fix test_ec2_list_credentials
fix test_ec2_list_credentials

The author used a wrong method to create ec2 credential and asserted
the wrong list data:
1. The method which creates a ec2 credential is self._get_ec2_cred()
2. cred_list should be r.result['credentials']

Change-Id: Ibb934ea404654adc99220096edf913a8aa8df7b3
Closes-Bug: #1413459

vivekdhayaal/keystone,ajayaa/keystone,rushiagr/keystone,openstack/keystone,roopali8/keystone,MaheshIBM/keystone,blueboxgroup/keystone,jumpstarter-io/keystone,takeshineshiro/keystone,ilay09/keystone,UTSA-ICS/keystone-kerberos,jumpstarter-io/keystone,UTSA-ICS/keystone-kerberos,dstanek/keystone,ilay09/keystone,dims/keystone,klmitch/keystone,ajayaa/keystone,mahak/keystone,rajalokan/keystone,dstanek/keystone,nuxeh/keystone,jonnary/keystone,rajalokan/keystone,maestro-hybrid-cloud/keystone,rajalokan/keystone,himanshu-setia/keystone,takeshineshiro/keystone,cernops/keystone,ilay09/keystone,dstanek/keystone,mahak/keystone,klmitch/keystone,promptworks/keystone,nuxeh/keystone,vivekdhayaal/keystone,jamielennox/keystone,idjaw/keystone,openstack/keystone,dims/keystone,vivekdhayaal/keystone,jumpstarter-io/keystone,rushiagr/keystone,cernops/keystone,jonnary/keystone,blueboxgroup/keystone,roopali8/keystone,idjaw/keystone,himanshu-setia/keystone,rushiagr/keystone,nuxeh/keystone,promptworks/keystone,maestro-hybrid-cloud/keystone,jamielennox/keystone,openstack/keystone,MaheshIBM/keystone,mahak/keystone,promptworks/keystone