Correct test_get_usage_tenant's AttributeError

When show_tenant_usage didn't get the expected result in VALID_WAIT
time(30 secs), we will get "AttributeError: 'TenantUsagesTestJSON'
object has no attribute 'resp'". This is to correct this and make
the error message more understandable.

Change-Id: Ic9e83b7c2cafbb27eed240f4bec15c1dbd5694a5
Closes-Bug: #1586319
1 file changed