Add plugin support to the installer

TLJH is a great base to build 'stacks' on top of
for various use cases. These 'stacks' should be built by
people who are domain experts in their fields, and easily
updateable with new TLJH versions. Extension points need
to be very clearly defined & evolvable, so we can modify
TLJH without fear of breaking everything.

[pluggy](https://pluggy.readthedocs.io/) is the plugin
mechanism for pytest spun out into its own library,
and fits our requirements well.

There is an experimental pangeo stack in progress at
https://github.com/yuvipanda/tljh-pangeo for an example
of how this would work

tljh/installer.py

@@ -2,15 +2,17 @@ import argparse
 import os
 import secrets
 import subprocess
+import itertools
 import sys
 import time
 import logging
 from urllib.error import HTTPError
 from urllib.request import urlopen, URLError
+import pluggy
 
 from ruamel.yaml import YAML
 
-from tljh import conda, systemd, traefik, user, apt
+from tljh import conda, systemd, traefik, user, apt, hooks
 from tljh.config import INSTALL_PREFIX, HUB_ENV_PREFIX, USER_ENV_PREFIX, STATE_DIR
 
 HERE = os.path.abspath(os.path.dirname(__file__))
@@ -305,6 +307,44 @@ def ensure_symlinks(prefix):
             return
     os.symlink(tljh_config_src, tljh_config_dest)
 
+def run_plugin_actions(plugins):
+    """
+    Run installer hooks defined in plugins
+    """
+
+    # Install plugins
+    if plugins:
+        conda.ensure_pip_packages(HUB_ENV_PREFIX, plugins)
+
+    # Set up plugin infrastructure
+    pm = pluggy.PluginManager('tljh')
+    pm.add_hookspecs(hooks)
+    pm.load_setuptools_entrypoints('tljh')
+
+    # Install apt packages
+    apt_packages = list(set(itertools.chain(*pm.hook.tljh_extra_apt_packages())))
+    if apt_packages:
+        logger.info('Installing {} apt packages collected from plugins: {}'.format(
+            len(apt_packages), ' '.join(apt_packages)
+        ))
+        apt.install_packages(apt_packages)
+
+    # Install conda packages
+    conda_packages = list(set(itertools.chain(*pm.hook.tljh_extra_user_conda_packages())))
+    if conda_packages:
+        logger.info('Installing {} conda packages collected from plugins: {}'.format(
+            len(conda_packages), ' '.join(conda_packages)
+        ))
+        conda.ensure_conda_packages(USER_ENV_PREFIX, conda_packages)
+
+    # Install pip packages
+    pip_packages = list(set(itertools.chain(*pm.hook.tljh_extra_user_pip_packages())))
+    if pip_packages:
+        logger.info('Installing {} pip packages collected from plugins: {}'.format(
+            len(pip_packages), ' '.join(pip_packages)
+        ))
+        conda.ensure_pip_packages(USER_ENV_PREFIX, pip_packages)
+
 
 def main():
     argparser = argparse.ArgumentParser()
@@ -317,11 +357,14 @@ def main():
         '--user-requirements-txt-url',
         help='URL to a requirements.txt file that should be installed in the user enviornment'
     )
+    argparser.add_argument(
+        '--plugin',
+        nargs='*',
+        help='Plugin pip-specs to install'
+    )
 
     args = argparser.parse_args()
 
-
-
     ensure_admins(args.admin)
 
     ensure_usergroups()
@@ -335,6 +378,9 @@ def main():
     ensure_jupyterhub_running()
     ensure_symlinks(HUB_ENV_PREFIX)
 
+    # Run installer plugins last
+    run_plugin_actions(args.plugin)
+
     logger.info("Done!")