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