Use __context__ to reduce calls to chkconfig/runlevel

This commit uses the __context__ dict to keep
salt.modules.rh_service._services() from running chkconfig --list and
runlevel every time it is called (which is often). The service.running
and service.dead states will clear the context data before trying to
detect changes.

This fixes #4295.
Merge pull request #4387 from terminalmage/issue4295

Use __context__ to reduce calls to chkconfig/runlevel