Rename omnibus build target to fedora-20.
Handle rename in the tests.
Name docker image consistently.

Use build- instead of build_ to match delimiter of distribution.
Merge remote-tracking branch 'origin/master' into omnibus-build-name-FLOC-1026
Update some remaining references to fedora20
Merge pull request #989 from ClusterHQ/omnibus-build-name-FLOC-1026

[FLOC-1026] Rename omnibus build target to fedora-20

Author: @tomprince 
Reviewer: @wallrj
Fixes https://clusterhq.atlassian.net/browse/FLOC-1026