Skip extend_start.sh if KOLLA_SKIP_EXTEND_START is defined

This is all kolla-mesos needs, so that we can run the extend_start
within the context of our start script. We need to do this to track
dependencies of the commands (one of which is the extend_start).

implements blueprint custom-extend-start

Change-Id: Ib844adc9c42dd8862281eb3f903d6c2b4695a34d