rename ACTIVE_PENDING to ACTIVE_PENDING_STATUSES

Looking at the lbaas code it's not very obvious that constants.ACTIVE_PENDING
is a list of statuses that contain statuses that are ACTIVE or
PENDING. This patch renames ACTIVE_PENDING to ACTIVE_PENDING_STATUSES so
it's obvious that this is a list and not just a string called ACTIVE_PENDING.

Closes-bug: #1295790

Change-Id: I7af96bcc6b145c6ab809c0b032ccb18baad9c98e
Merge "rename ACTIVE_PENDING to ACTIVE_PENDING_STATUSES"