Separate token client from identity client

Identity 'TokenClientJSON' is being derived from 'IdentityClientJSON'
which is not needed. 'TokenClientJSON' class can be directly derived from
ServiceClient.

This patch makes the above inheritance changes and separate token
clients (V2 & v3) in separate file.

This will helpful in moving services client to tempest-lib.

Change-Id: I25186dc1fde57cd7fd16784aa6baf2f8d2f0a46d
10 files changed