Handle NetworkInUse exception in api layer

In nova-network, if we delete an existing network with port already
associated, a 500 error will be reported and this is not helpful for
end user. This patch catch the network in use exception and raise
an HTTPConflict exception with formatted message.

Change-Id: I7361e461e0eecc0ef6e47fa8ef9a48a24b7d5dfd
Closes-Bug: #1294920
Fix state mutation in cells image filter

Stop set pop() operations in the cells ImagePropertiesFilter from
mutating capability state.  This causes a race that can break the
filter between requests.

Closes-Bug #1325705

Change-Id: I901e39046a38ab60c23ddc8762ef5d19a297935b
Merge "Fix state mutation in cells image filter"
Merge "Handle NetworkInUse exception in api layer"