Refactor ResourceGroup's FnGetAtt()

Change-Id: I424e4ce67bbc04739e028dfbe197595e4c6254f8
Remove tools/glance-jeos-add-from-github.sh

glance-jeos-add-from-github.sh has not been edited since 2012-08-20, and
its image links have been broken (according to Google cache) for a
minimum of 9 months.

The functionality of the script seems to be superceded by the
handy-dandy diskimage-builder tool.

See http://docs.openstack.org/developer/heat/getting_started/jeos_building.html
for more info about diskimage-builder workflow.

Change-Id: I99880f1cca4af84fa7193d749a7c8f6957bb0b78
Partial-Bug: 1338066
Move network constraint to neutron plugin

This allows exception imports to be encapsulated inside the neutron
plugin, and plugins are likely the most appropriate place for
custom constraints to live.

Change-Id: I5766edb305b7cfffdcd8943fb6edce7f3e9ff26d
Move glance_utils functions to glance client plugin

The glance client plugin is a more obvious home for this sort
of function, and this change makes mocking of these functions
much easier since a mocked client is no longer needed.

To access the methods defined in the client plugin the method
Resource.client_plugin has been added. When all clients are
contributed by plugins then method Resource.client will call
Resource.client_plugin.

glance_utils can be deleted since it was only added at the
beginning of the Juno cycle.

The image constraint has also been moved to the glance plugin.

Change-Id: Id31ccd605411ca034addc67d8aadf7cea0904bfd
Merge "Move network constraint to neutron plugin"
Merge "Remove tools/glance-jeos-add-from-github.sh"
Merge "Refactor ResourceGroup's FnGetAtt()"
Merge "Move glance_utils functions to glance client plugin"