heat_keystoneclient migrate create_ec2_keypair to v3 API

Convert create_ec2_keypair to use the v3/credentials API, which
is the v3-compatible way to manage ec2 keypairs.

NOTE: requires the fix for keystone bug #1259584 (now in keystone
master) or signature validation with these credentials will fail.

Change-Id: I9fbc1611c53cd14177b5c0dfe25a96a3922fd1cc
blueprint: keystone-v3-only
Store credential ID for AccessKey and SignalResponder

These resources create an ec2 keypair, which when managed via the
keystone v3/credentials should be accessed via credential ID not
the access key ID, so store the credential ID for performing any
operations (get/update/delete) on the credential, as it's preferable
to having to do a brute-force lookup by access key (no way to filter
on the access key via v3/credentials)

Change-Id: I5995fb003c998fc2d7f331c6da7430adcab168b2
blueprint: keystone-v3-only
Merge "heat_keystoneclient migrate create_ec2_keypair to v3 API"
Merge "Store credential ID for AccessKey and SignalResponder"