Remove unnecessary steps for cold snapshots

Up until now when we created cold snapshots we were stopping the
instance, create an internal snapshot, extract the snapshot to a file
and then delete the internal snapshot before bringing up the instance.

If the instance is shut down, there's no concurrent writer, so the image
can be directly extracted without taking an internal snapshot first,
because the snapshot and the current state are the same.

In this patch the creation and deletion of the internal snapshot are
removed to eliminate the extra steps and optimize the creation of
snapshots a bit.

Closes-Bug: #1242819
Closes-Bug: #1247185

Change-Id: I429fa2e1e26aea321eecbf14edd11263fff525ae
(cherry picked from commit 8c985874c7885f31871204d3f83ce547fefc5fb6)
Merge "Remove unnecessary steps for cold snapshots" into stable/havana