Set issuer value to CONF.saml.idp_entity_id.

When generating SAML assertion Keystone should always set issuer value
in federation.controllers.Auth.create_saml_assertion() to
CONF.saml.idp_entity_id.

Change-Id: If970cdf20cfca8b1dc667eefd030083fdafe9424
Closes-Bug: #1374033
Merge "Set issuer value to CONF.saml.idp_entity_id."