Deploy locks

Add a deploy lock for commands that are unsafe to run concurrently.

The lock is taken by creating a `mrsk_lock` directory on the primary
host. Details of who took the lock are added to a details file in that
directory.

Additional CLI commands have been added to manual release and acquire
the lock and to check its status.

```
Commands:
  mrsk lock acquire -m, --message=MESSAGE  # Acquire the deploy lock
  mrsk lock help [COMMAND]                 # Describe subcommands or one specific subcommand
  mrsk lock release                        # Release the deploy lock
  mrsk lock status                         # Report lock status

Options:
  -v, [--verbose], [--no-verbose]                # Detailed logging
  -q, [--quiet], [--no-quiet]                    # Minimal logging
      [--version=VERSION]                        # Run commands against a specific app version
  -p, [--primary], [--no-primary]                # Run commands only on primary host instead of all
  -h, [--hosts=HOSTS]                            # Run commands on these hosts instead of all (separate by comma)
  -r, [--roles=ROLES]                            # Run commands on these roles instead of all (separate by comma)
  -c, [--config-file=CONFIG_FILE]                # Path to config file
                                                 # Default: config/deploy.yml
  -d, [--destination=DESTINATION]                # Specify destination to be used for config file (staging -> deploy.staging.yml)
  -B, [--skip-broadcast], [--no-skip-broadcast]  # Skip audit broadcasts
```

If we add support for running multiple deployments on a single server
we'll need to extend the locking to lock per deployment.

test/cli/main_test.rb

@@ -42,12 +42,14 @@ class CliMainTest < CliTestCase
     Mrsk::Cli::Main.any_instance.expects(:invoke).with("mrsk:cli:prune:all", [], invoke_options)
 
     run_command("deploy", "--skip_push").tap do |output|
+      assert_match /Acquiring the deploy lock/, output
       assert_match /Ensure curl and Docker are installed/, output
       assert_match /Log into image registry/, output
       assert_match /Pull app image/, output
       assert_match /Ensure Traefik is running/, output
       assert_match /Ensure app can pass healthcheck/, output
       assert_match /Prune old containers and images/, output
+      assert_match /Releasing the deploy lock/, output
     end
   end