Added snapshot support for image build&push

Now user can set in config/settings.yml file for if she wants use
snasphot mode for providers for which imagefactory supports snapshots.
Currently these are: ec2, openstack, mock

This patch also removes the old annoying restriction when provider name
was used for finding realm on imagefactory side.

Fixes #276
Merge pull request #378 from jprovaznik/snapshot1

Added snapshot support for image build&push
remove all foreign keys until rails is fixed
https://github.com/rails/rails/pull/8548
Merge pull request #391 from martinpovolny/revert-fks

remove all foreign keys until rails is fixed
BZ 870479 - fixed checking of "stopped after creation" state

For rhevm/vsphere an instance goes to 'stopped' state after creation so
conductor has to send explicit start request. To distinguish "stopped after
creation" state and common "stopped" state conductor checks if last
instance launch request was sent after last deployment launch request
(there can be multiple launch requests for deployment/instance because of
rolblack+retry process).

In some cases it was possible that instance launch request was picked up by
delayed_job in same second when the deployment was created, so checking create
times by seconds was not precise enough.
Merge pull request #392 from jprovaznik/stop_fix

BZ 870479 - fixed checking of "stopped after creation" state