heat_keystoneclient convert delete_ec2_keypair to v3 API

Convert delete_ec2_keypair to v3/credentials API, with code to
handle backwards compatibility where existing resources have only
the access ID stored in the DB.

Change-Id: Ib1cad738674fa9441410af0927ce8f2fc204225e
blueprint: keystone-v3-only
Fix user and signal responder exception import

Both user and signal_responder indirectly import keystoneclient
exceptions via heat_keystoneclient.  Instead we should directly
import from keystoneclient, otherwise this relies on the internal
import naming in heat_keystoneclient.

Change-Id: Iff6570f890024a04295b234255c63e2ba1433cb4
Merge "heat_keystoneclient convert delete_ec2_keypair to v3 API"
Merge "Fix user and signal responder exception import"