Change unit test for availability_zones.reset_cache

Test_rest_availability_zone_reset_cache can not make sense
because the result of az._get_cache().get('cache') before
executing az.reset_cache() and after az.reset_cache() are
same.

This Patch will provide a reasonable test for it.

Change-Id: I7a27998bdf70f39077a5d79fc460974fc5806c3e
Closes-bug: #1258041
Merge "Change unit test for availability_zones.reset_cache"