Make deleting transient clusters safe

We want Sahara to be able to run distributed. I.e. several sahara-api
and sahara-engine should run in parallel and the whole setup should
work fine. For that to work we need to make deleting periodic clusters
safe, since a cluster could be deleted by two engines simultaneously.

Partially Implements: blueprint scalable-savanna
Change-Id: I33f9813ba58a4cca7d0221f518c29f8e605cc999
Merge "Make deleting transient clusters safe"