tests: factor out api calls response parsing

This will prevent unclear KeyError Exceptions when the service has failed to
load or when an api call has raised an exception.

Change-Id: I2355f3a462ccbaff768f2555d6e829c7861eb74a
Signed-off-by: ibarkan <ibarkan@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/35955
Reviewed-by: Dan Kenigsberg <danken@redhat.com>