Don't set context auth_token in heat_keystoneclient

Now that all clients refer directly to the auth_token in the auth_ref
there's no need to set the auth_token in the context when consuming
a trust.

Change-Id: I2fe58de7b950a65672b490f1769c88dedda3895b
Partial-Bug: #1306294
Merge "Don't set context auth_token in heat_keystoneclient"