Remove sample yum_content plugin

It will re emerge as ostree plugin in
src/content_plugins/ostree
Add content_plugins dir,and ostree content plugins
HACK: install ostree content plugin in site-packages

/usr/share/rhsm-plugins/ is not on sys.path, so without
append to path, we can't include submodules there.

Plugins can install their helper modules to standard
python paths. At the moment, hardcode
/usr/lib/python2.7/site-packages/rhsm_content_plugins

Need to generalize that. Plugins may need their
own install process. But this get's things testable.
Add ostree_content plugin entry point

It's modules can live in site-packages
Log exceptions from trying to load plugins.
Make ostree content action loaded from plugins.
Remove unneeded #!/usr/bin/python

This module is not invoked as a main, only
imported.
Move OstreeContentActionInvoker to it's own module
Add TODO about content_plugin installs

To install content plugins:
   make install-content-plugins
   make install-content-plugins-conf

src/content_plugins/ostree_content.py is the ostree_content
plugin entry point.

ostree_content specific modules can live in
src/content_plugins/ostree/. That dir gets installed to
/usr/lib/python2.7/site-packages/rhsm_content_plugins/ostree/

(yes, hardcoded atm)