Add rule overwrite flag to Enforcer class

* Add a flag to policy.Enforcer to control if rules read from policy
config file overwrite existing rule(s) in Enforcer object.

This requirement comes from Glance adoption. In Glance domain model,
before load policy configure file(s) (if needed - force or file update)
ProtectProperty logic domain provided may already added some rules
according to operator configure, so policy.Enforcer need to keep them
there instead of always overwrite them by configure files in any case.

* Fix a minor issue which causes use_conf flag loss once load_rules()
loaded rules from configure file(s), without this flag, when a configure
file got changed, reloading will not be triggered when calling
enforcer().

Change-Id: I6f552dcf37128668abc71f5804ff12902a0e1850
Signed-off-by: Zhi Yan Liu <zhiyanl@cn.ibm.com>