Disable nested validation for ResourceGroup with zero count

Some users (TripleO specifically) want to disable features via a
count of zero, which is a problem as we always recurse and validate
the nested stack since c31c34f8dfd0919bf46a975701c139073115debc

Instead, we only do validation when the count is non-zero, to
enable, e.g default image names, to be ignored at validation
time (as we'll never use them) instead of rejected by the nested
schema (e.g the server.py properties schema which contains a
custom contraint to always validate the image).

This should still allow us to fail fast (at validation time before
creating anything) when we're actually about to create something,
e.g when the count is non-zero.

Change-Id: I411ff41a9e0730e9864f5ed4ac54f1d5d0ec02d7
Closes-Bug: #1401929
Remove gettextutils import

Change-Id: I48846926cc5eb2a1f81db6a2becb310bdf61daa7
Closes-Bug: #1404206
Fix doc generation for contrib resources

Since we shouldn't require contrib resources to be installed in order to
generate documentation, this adds an exception to the plugin loader so
that it doesn't try to load the setup.py files that can be found inside
each plugin directory inside contrib/

Closes-Bug: 1403897
Change-Id: Id6ece4379599925dd17d22cef401c8d667a0ac51
Re-enable doc generation for contrib plugins

This re-enables doc generation for contrib plugins and removes legacy
code no longer needed because of stevedore.

Related-Bug: 1403897
Change-Id: Ia51cb7c8b0c375d2a430d45b9ece50eb67271788
Merge "Disable nested validation for ResourceGroup with zero count"
Merge "Fix doc generation for contrib resources"
Merge "Remove gettextutils import"
Merge "Re-enable doc generation for contrib plugins"