share neutron admin auth tokens

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_ admin
nova.network.neutronv2 calls in a 24-hour period.

DocImpact
Signed-Off-By: Peter Feiner <peter@gridcentric.ca>
Closes-Bug: 1255594

Change-Id: I5c743aa86030e4ab000dcdefb6a7183305512303