Change import of exceptions for keystoneclient

Keystoneclients uses module `keystoneclient.exceptions` for storing
exceptions, so it needed to import `keystoneclient.exceptions`
instead of `keystoneclient.apiclient.exceptions` in tempest.

Change-Id: I68cb50bd092e9cc304710bf7748a1c0de5554612
Closes-Bug: #1276093
Merge "Change import of exceptions for keystoneclient"