Make sure report_interval is less than service_down_time

Services that inherit service.py/Service class would register
themselves to DB and then update stats periodically (every
report_interval second).  The consumer of this kind of information,
like scheduler or 'os-service' API extension, will consider a service
is 'up' (active) if last update from that service is not longer than
'service_down_time' ago.

The problem is if 'report_interval' was configured/provided greater
than 'service_down_time' by mistake, services would then be always
considered in 'down' state, which can result in unsuccesful placement
of volume create request for example.  This is what Bug #1255685 is
about.

In previous fix: https://review.openstack.org/#/c/60760/, a
configuration check helper function basic_config_check() was added
*wrongly* to WSGIService class instead of Service class.  This patch
moves the configuration check helper function and the check to the
right place to make sure 'report_interval' is less then
'service_down_time'.

Closes-bug #1255685

Change-Id: I14bd8c54e5ce20719844f437808ad98a011820de