Change rest client exceptions' inheritances

RateLimitExceeded, ServerFault and OverLimit raise at rest_client layer
and they should inherit from RestClientException for representing what
happens clearly. This patch changes them.

Change-Id: I0b74bd44a88cc68bdaeab6bd605722d47f5a28a9
diff --git a/tempest/exceptions.py b/tempest/exceptions.py
index c30d4c5..213d5de 100644
--- a/tempest/exceptions.py
+++ b/tempest/exceptions.py
@@ -140,15 +140,15 @@
     message = "Endpoint not found"
 
 
-class RateLimitExceeded(TempestException):
+class RateLimitExceeded(RestClientException):
     message = "Rate limit exceeded"
 
 
-class OverLimit(TempestException):
+class OverLimit(RestClientException):
     message = "Quota exceeded"
 
 
-class ServerFault(TempestException):
+class ServerFault(RestClientException):
     message = "Got server fault"