keystone : Use Ec2Signer utility class from keystoneclient

The Ec2Signer class has been moved to python-keystoneclient,
so we can remove the internal implementation and import the
keystoneclient version

blueprint ec2signer-to-keystoneclient

Change-Id: I19d8575ab8b972467ce280a6197ae762da1ce790
Merge "keystone : Use Ec2Signer utility class from keystoneclient"