share neutron admin auth tokens

NOTE: Test is a mess since rebasing. Needs to be cleaned up like v1
of this patch.

Admin tokens in nova.network.neutronv2 were being created once per
"thread" (i.e., green thread) and shared. For example, during instance
creation, a single token was created then reused 6 times.  However,
this scheme was far from optimal. Given that tokens, by default, have
a shelf life of 24H, a single token can be shared by _all_
nova.network.neutronv2 calls in a 24-hour period.

Change-Id: I5c743aa86030e4ab000dcdefb6a7183305512303
Closes-Bug: 1255594