Override partial installs by default - part three (#4331)

* During install, remove prior unfinished installs

If a user performs an installation which fails, in some cases the
install prefix is still present, and the stage path may also be
present. With this commit, unless the user specifies
'--keep-prefix', installs are guaranteed to begin with a clean
slate. The database is used to decide whether an install finished,
since a database record is not added until the end of the install
process.

* test updates

* repair_partial uses keep_prefix and keep_stage

* use of mock stage object to ensure that stage is destroyed when it should be destroyed (and otherwise not)

* add --restage option to 'install' command; when this option is not set, the default is to reuse a stage if it is found.

var/spack/repos/builtin.mock/packages/canfail/package.py

@@ -0,0 +1,43 @@
+##############################################################################
+# Copyright (c) 2013-2016, Lawrence Livermore National Security, LLC.
+# Produced at the Lawrence Livermore National Laboratory.
+#
+# This file is part of Spack.
+# Created by Todd Gamblin, tgamblin@llnl.gov, All rights reserved.
+# LLNL-CODE-647188
+#
+# For details, see https://github.com/llnl/spack
+# Please also see the LICENSE file for our notice and the LGPL.
+#
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU Lesser General Public License (as
+# published by the Free Software Foundation) version 2.1, February 1999.
+#
+# This program is distributed in the hope that it will be useful, but
+# WITHOUT ANY WARRANTY; without even the IMPLIED WARRANTY OF
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the terms and
+# conditions of the GNU Lesser General Public License for more details.
+#
+# You should have received a copy of the GNU Lesser General Public
+# License along with this program; if not, write to the Free Software
+# Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
+##############################################################################
+from spack import *
+
+
+class Canfail(Package):
+    """Package which fails install unless a special attribute is set"""
+
+    homepage = "http://www.example.com"
+    url      = "http://www.example.com/a-1.0.tar.gz"
+
+    version('1.0', '0123456789abcdef0123456789abcdef')
+
+    def install(self, spec, prefix):
+        try:
+            succeed = getattr(self, 'succeed')
+            if not succeed:
+                raise InstallError("'succeed' was false")
+            touch(join_path(prefix, 'an_installation_file'))
+        except AttributeError:
+            raise InstallError("'succeed' attribute was not set")