Enforce FlavorExtraSpecs Key format

When creating a key name with characters like '/' that are not
being encoded or cannot be handled as expected by routes when
matching the url, then it's not possible to remove them anymore.
The new format apply the restriction: r"^[\w\.\-: ]+$"

Change-Id: Ie6f982a0e44212bf0d58da66b89aff7740d06e8f
Closes-Bug: #1256119
Merge "Enforce FlavorExtraSpecs Key format"