Make DbPlugin accept status in create_network().

This commit makes NeutronDbPluginV2 get network status from input at
create_network() to allow other plugins to set network status while
creating DB entry.

Now plugins can create network with any status in a single DB commit.
(e.g. Set network status to 'DOWN' when admin_state_up is False.)

This commit also changes NeutronDbPluginV2TestCase() to make default
network status configurable in each plugin test case.

Fixes: bug #1207167
Change-Id: Id3ad26719036c631e4b268150407a2c69919203f
Merge "Make DbPlugin accept status in create_network()."