Add RateLimitExceeded test case
Add RateLimitExceeded test case

The rest_client module raises RateLimitExceeded exception, but
there was not any tests for the exception. This patch adds it.

Change-Id: I933ebe2ed52750088e3941b308b669f088d7f8ed

sorenh/tempest-lib,varunarya10/tempest-lib,afaheem88/tempest-lib,JioCloud/tempest-lib,cneill/tempest-lib