Local healthcheck

This is way faster than the normal one, so we can get faster feedback

Changed the health check script to be a command line script rather than
a rake task as it's easier to accept and parse command line options.
    
I've used a gem called 'slop' for command line parsing:
      https://github.com/injekt/slop

Health check supports multiple backend types. This should mean we can 
delete the govuk-search-health repo.