Add praefect check to praefect startup

praefect has a subcommand check eg: praefect -config <config_file> check that will go through a series of checks to make sure the cluster is healthy. We want this to run on praefect's startup, and for runit to not start praefect if it's not healthy (otherwise it will simply flap)

However, we also want the ability to start/restart praefect without the checks since they can possible take a long time.

We can use the omnibus-ctl's pre-hook api to accomplish this. In the pre-hook, we can check for the existence of --skip-check in ARGV. If it exists, then we can set an environment variable PRAEFECT_CHECK_SKIP_CHECKS which then the praefect check subcommand can read and if set to true, simply exit with a 0.

The issue for the praefect subcommand change is here: gitaly#3851 (closed)