Added join_cluster param to have consul join a cluster after (re)starting service

For our use case we are bootstrapping environments and needed a way to designate certain roles to act as servers, and to have them all join the cluster based on a DNS deployment convention.  This PR does 2 things:

1. To be able to elect a leader, added the bootstrap_expect config as a simple integer cast in config.json.erb, until a more advanced approach can be implemented as described in PR 13.
2. Added a param, join_cluster and an exec resource that subscribes to the consul service to re-join the cluster after any service restart.
There is a bit of complexity here, I would like to see a test to ensure the right thing is happening, and will continue in the future.

How about 
```
context 'The bootstrap_expect in config_hash is an int' do
  let(:params) {{
    :config_hash =>
      { 'bootstrap_expect' => '5' }
  }}
  it { should contain_file('config.json').with_content(/bootstrap_expect: 5/) }
  it { should_not contain_file('config.json').with_content(/bootstrap_expect: "5"/) }
end
```