Modifed http_check to init keystone clients and re-use
Modifed http_check to init keystone clients and re-use

The previous code was initializing the keystone client each
time check was called in the case use_keystone is True.
Our default http_check config uses drop wizard checks that do
not require keystone.  This could eliminate the number of
keystone token requests for any http_checks that require
a keystone token, since now they will be cached.

Change-Id: I47827d549365c29a36846d51ede0fb06da435ac7

sapcc/monasca-agent,sapcc/monasca-agent,sapcc/monasca-agent