Using sql as default driver for tokens

kvs driver for tokens is not a production quality storage method.
The shortcomings of using kvs as storage driver for tokens:
    1. It requires load balancer to persist connections to a single
keystone server by token.
    2. The memory will grow out of control until token_flush is run.
    3. At some point kvs lookups get very slow because there are millions
of keys in the dict.
    4. Process restart invalidates all tokens.

Fixes: bug #1188370
Change-Id: Ic726e12d798b843412158a7b92f5e3e3a654811f
Merge "Using sql as default driver for tokens"