heat api : Update ec2token middleware for v4 signatures

Update ec2token so it can verify v4 signature formats.

Note for v4 signatures to work you need keystone to be using
python-keystoneclient >= 0.2.4, or contain the patch from
https://review.openstack.org/#/c/26013/

This change is backwards compatible, as older keystoneclient
versions will simply ignore the additional request keys

fixes bug #1122472

Change-Id: Iccc6be7913ab5ca5813a2e0c8f66cda0ccd85a0b