Each config option has limitation for type and value.
In production code, oslo.conf can ensure user's input
is valid, but in unit test, test methods can pass if
we use method CONF.set_override without parameter
enforce_type=True even we pass wrong type or wrong
value to config option. This commit makes sure calling
method CONF.set_override with enforce_type=True.
Closes-bug: #1517839
Change-Id: If01841b30c10e994028dcdefbb389425cb0facb1
9f525c098f