NSX sync cache: add a flag to skip item deletion

This new flag will prevent updates on single resources, which
can happen asynchronously to the main synchronization thread,
from processing the 'changed' flag for items in the cache and
delete items not present anymore in NSX.

This will avoid failures seen in CI runs where the sync
thread failed because of a KeyError because some items where
concurrently removed from the NSX cache.

Change-Id: I6a11b8dabb1e9481dc807b5fbba17c9027dad5f2
Closes-Bug: #1329650
(cherry picked from commit bd4ef02dc12122ad019ada803fd1a2bb2c56a5a2)
Merge "NSX sync cache: add a flag to skip item deletion" into stable/icehouse
Merge branch 'stable/icehouse' of https://github.com/openstack/quantum into stable/icehouse