Make the wait_for_server_status timeout message a bit more clear

The current error message when wait_for_server_status times out only
includes the vm_state but the method, by default, is really waiting for
a state combination of vm_state/task_state.

Sometimes the vm_state is the desired status but the task_state is not,
for example:

"Server 613b9fd8-9f3f-41a6-86cc-a36301aa7fc8 failed to reach ACTIVE
status within the required time (196 s). Current status: ACTIVE."

This patch adds the current task_state to the error message for clarity
when the timeout occurs.

Related-Bug: #1257561

Change-Id: Icfce12035bf4938eac3a13a7b640db4398ef832f
Wait for backup images to be ACTIVE in test_create_backup

The test_create_backup test creates two backup snapshot images of the
instance right away and waits for the instance to be ACTIVE before
continuing each time, and before creating the third and final backup
image.

The problem is the instance is always ACTIVE, it's the task state that
is transitioning on the instance while it's creating backup snapshot
instances and checking for old rotations to delete.

The test needs to not only check that the instance is ACTIVE before
creating each new backup but also that the snapshot image backups
themselves are ACTIVE which is a better indication that the instance's
state transition is complete.

Closes-Bug: #1257561

Change-Id: I5b423b3424e1dfe6cf5f8fef0dc9538961c1f28d
Merge "Make the wait_for_server_status timeout message a bit more clear"
Merge "Wait for backup images to be ACTIVE in test_create_backup"