core, webadmin: Warn about incompatible upgrade

When upgrading a DC, a warning is issued if this upgrade would cause
upgrading the domains' storage format.

This patch improves that message by adding information that this upgrade
will cause the domains to be unusable with products of version XYZ.

The patch includes:
1. A method in VersionStorageFormatUtil to retrieve the earliest
   supported version of a given format.
2. A unit test for said method
3. Migrating the message from UIConstants to UIMessages, and adding a
   version parameter.
4. The call in DataCenterListModel for said message.

Change-Id: I3fa7345caad8603c03fb042dec2cebf6fdc03f44
Bug-Url: https://bugzilla.redhat.com/957939
Signed-off-by: Allon Mureinik <amureini@redhat.com>