Added Identity API tests for user ec2 credentials

The Identity API is not covered in a whole for non-admin user.
Since user can manipulate ec2 credentials, created tests for
creating, listing, getting, deleting ec2 credentials.
Added tests:
* test_create_ec2_credentials
* test_list_ec2_credentials
* test_show_ec2_credentials
* test_delete_ec2_credentials.

Change-Id: I9112916aa744a0a7522330d12a7cf410be28dbf6
4 files changed