LBaaS: Fix incorrect pool status change

Avoid incorrect status change when deleting the pool.
We can check for the conditions prior putting the pool
to PENDING_DELETE state, in case delete conditions are met
it is safe to change the state to PENDING_DELETE.
Also, change create_vip and update_vip operations to respect
PENDING_DELETE and avoid race conditions.

Change-Id: I9f526901eb85bdb83cf4ff8131460eb592c900f8
Closes-Bug: #1242338
Merge "LBaaS: Fix incorrect pool status change"