Fix concurrency issue with HeatStack.push

All changes to template made with call HeatStack.updateTemplate
while HeatStack.push is in progress will be lost after HeatStack.push
will be finished.

Co-Authored-By: Stan Lagun <slagun@mirantis.com>
Closes-bug: #1370503
Change-Id: Ia5c0457a2841bbcb30bc60f5ab99f7872a77b51e
Merge "Fix concurrency issue with HeatStack.push"