Allow v2 client authentication with trust_id

It should be possible to authenticate against the v2 tokens
API with a trust_id, because it suports rescoping an existing
token to a trust, this patch adds client support for this.

Note with the current keystone code it's necessary to pass the
trustor tenant ID when rescoping with a trust where
impersonation==True, e.g:

c = client_v2.Client(username=TRUSTEE_USERNAME,
                     password=TRUSTEE_USERNAME,
                     tenant_name=TRUSTEE_TENANT_NAME,
                     auth_url=OS_AUTH_URL_V2)
c.authenticate(trust_id=trust_i.id, tenant_id=TRUSTOR_TENANT_ID)

Change-Id: I177c41af298b7437e2c6fb437aa9ce9a09773b9d
Closes-Bug: #1231483
Merge "Allow v2 client authentication with trust_id"