presence: Only allow loading once

If entity manager were to get restarted, the JsonConfig::start()
function would be called again which would attempt to load a config on
top of an existing one.

Fix that by only allowing the config to load once.  If someone wanted
the config to load again, they could either send the HUP signal or just
restart the service.

Signed-off-by: Matt Spinler <spinler@us.ibm.com>
Change-Id: I1526db17500ffe88408c8bc1f0abba9c4952107b
2 files changed