Cache Neutron Client for Admin Scenarios

Store a thread local copy of Neutron client for use with admin
tokens, this will prevent excessive calls to keystone to mint
new tokens and will enable reuse of the already set up client
for subsequent calls in this thread.

Change-Id: I6346281ca3516c31b195ce71942786d724e5e9df
Closes-Bug: #1250580
Merge "Cache Neutron Client for Admin Scenarios"