Move implementation_signature to StackResource

We need to use common logic for the update determination which
uses implementation_signature, so move the common code from
TemplateResource to the StackResource base-class.

Change-Id: Ibd64b3607721487be5b0562cdb54e6c76db51149
Partial-Bug: #1389499
(cherry picked from commit f5221feb8bf9066b1a9cf078f224f8b00cacb5fc)
Merge "Move implementation_signature to StackResource" into stable/juno