heat_keystoneclient convert get_ec2_keypair to v3 API

Convert get_ec2_keypair to use v3/credentials interface, with
backwards compatible support for getting credentials stored via
the ec2tokens API (where we'll only have the access ID stored)

NOTE: requires the fix for keystone bug #1269637 (now in keystone
master).

Change-Id: I55515129b75566335bea24b7ed5246a34a15a747
blueprint: keystone-v3-only
Merge "heat_keystoneclient convert get_ec2_keypair to v3 API"