Rewrite bootstrapper in Python

- This was going to get too complex for bash. Only way to
  kill those scripts is before they get too complex.
- Better progress messages from bootstrapper.
- Differentiate between bootstrapper & installer
- Cleanup documentation a little bit

bootstrap/bootstrap.py

@@ -0,0 +1,125 @@
+"""
+Bootstrap an installation of TLJH.
+
+Sets up just enough TLJH environments to invoke tljh.installer.
+
+This script is run as:
+
+    curl <script-url> | sudo python3 -
+
+Constraints:
+  - Be compatible with Python 3.4 (since we support Ubuntu 16.04)
+  - Use stdlib modules only
+"""
+import os
+import subprocess
+import urllib.request
+import contextlib
+import hashlib
+import tempfile
+
+
+def md5_file(fname):
+    """
+    Return md5 of a given filename
+
+    Copied from https://stackoverflow.com/a/3431838
+    """
+    hash_md5 = hashlib.md5()
+    with open(fname, "rb") as f:
+        for chunk in iter(lambda: f.read(4096), b""):
+            hash_md5.update(chunk)
+    return hash_md5.hexdigest()
+
+
+def check_miniconda_version(prefix, version):
+    """
+    Return true if a miniconda install with version exists at prefix
+    """
+    try:
+        return subprocess.check_output([
+            os.path.join(prefix, 'bin', 'conda'),
+            '-V'
+        ]).decode().strip() == 'conda {}'.format(version)
+    except (subprocess.CalledProcessError, FileNotFoundError):
+        # Conda doesn't exist, or wrong version
+        return False
+
+
+@contextlib.contextmanager
+def download_miniconda_installer(version):
+    md5sums = {
+        '4.5.4': "a946ea1d0c4a642ddf0c3a26a18bb16d"
+    }
+
+    if version not in md5sums:
+        raise ValueError(
+            'minicondaversion {} not supported. Supported version:'.format(
+                version, ' '.join(md5sums.keys())
+            ))
+
+    with tempfile.NamedTemporaryFile() as f:
+        installer_url = "https://repo.continuum.io/miniconda/Miniconda3-{}-Linux-x86_64.sh".format(version)
+        urllib.request.urlretrieve(installer_url, f.name)
+
+        if md5_file(f.name) != md5sums[version]:
+            raise Exception('md5 hash mismatch! Downloaded file corrupted')
+
+        yield f.name
+
+
+def install_miniconda(installer_path, prefix):
+    subprocess.check_output([
+        '/bin/bash',
+        installer_path,
+        '-u', '-b',
+        '-p', prefix
+    ], stderr=subprocess.STDOUT)
+
+
+def pip_install(prefix, packages, editable=False):
+    flags = '--no-cache-dir --upgrade'
+    if editable:
+        flags += '--editable'
+    subprocess.check_output([
+        os.path.join(prefix, 'bin', 'python3'),
+        '-m', 'pip',
+        'install', '--no-cache-dir',
+    ] + packages)
+
+
+def main():
+    install_prefix = os.environ.get('TLJH_INSTALL_PREFIX', '/opt/tljh')
+    hub_prefix = os.path.join(install_prefix, 'hub')
+    miniconda_version = '4.5.4'
+
+    print('Checking if TLJH is already installed...')
+    if not check_miniconda_version(hub_prefix, miniconda_version):
+        initial_setup = True
+        print('Downloading & setting up hub environment...')
+        with download_miniconda_installer(miniconda_version) as installer_path:
+            install_miniconda(installer_path, hub_prefix)
+        print('Hub environment set up!')
+    else:
+        initial_setup = False
+        print('TLJH is already installed, will try to upgrade')
+
+    if initial_setup:
+        print('Setting up TLJH installer...')
+    else:
+        print('Upgrading TLJH installer...')
+
+    pip_install(hub_prefix, [
+        os.environ.get('TLJH_BOOTSTRAP_PIP_SPEC', 'git+https://github.com/yuvipanda/the-littlest-jupyterhub.git')
+    ], editable=os.environ.get('TLJH_BOOTSTRAP_DEV', 'no') == 'yes')
+
+    print('Starting TLJH installer...')
+    os.execl(
+        os.path.join(hub_prefix, 'bin', 'python3'),
+        os.path.join(hub_prefix, 'bin', 'python3'),
+        '-m',
+        'tljh.installer'
+    )
+
+if __name__ == '__main__':
+    main()