General enable/disable support

We have some enable/disable support in a number of workers, but many don't have it, such as the RubocopChecker, which is requested in #244. Also, the enable/disable support is a little inconsistent, so I'm thinking something more generic across the board would be very useful.

Additionally, this might allow us to separate out any private things (such as where the bot needs access to behind-the-firewall resources) from public things (such as where the bot may want to respond to a webhook), and allow us to run 2 bot instances (one internal and one external).

I'm thinking the configuration could be something similar to how rubocop itself works. So, within the bot we specify sensible defaults for most repos:

AllWorkers:
  Enabled: true
SqlMigrationChecker:
  Enabled: false

Then there can optionally be an .miq-bot.yml file within each repo where they can choose to enable or disable a specific worker (or in the future provide other options that can be overridden). So, for example, in manageiq-ui-service, they don't need the RubocopChecker at all since their linting is all done in Travis. Thus, they might have a file like:

RubocopChecker:
  Enabled: false