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.

lib/mrsk/cli/base.rb

@@ -6,6 +6,8 @@ module Mrsk::Cli
   class Base < Thor
     include SSHKit::DSL
 
+    class LockError < StandardError; end
+
     def self.exit_on_failure?() true end
 
     class_option :verbose, type: :boolean, aliases: "-v", desc: "Detailed logging"
@@ -71,5 +73,35 @@ module Mrsk::Cli
       def audit_broadcast(line)
         run_locally { execute *MRSK.auditor.broadcast(line), verbosity: :debug }
       end
+
+      def with_lock
+        acquire_lock
+
+        yield
+      ensure
+        release_lock
+      end
+
+      def acquire_lock
+        if MRSK.lock_count == 0
+          say "Acquiring the deploy lock"
+          on(MRSK.primary_host) { execute *MRSK.lock.acquire("Automatic deploy lock", MRSK.config.version) }
+        end
+        MRSK.lock_count += 1
+      rescue SSHKit::Runner::ExecuteError => e
+        if e.message =~ /cannot create directory/
+          invoke "mrsk:cli:lock:status", []
+        end
+
+        raise LockError, "Deploy lock found"
+      end
+
+      def release_lock
+        MRSK.lock_count -= 1
+        if MRSK.lock_count == 0
+          say "Releasing the deploy lock"
+          on(MRSK.primary_host) { execute *MRSK.lock.release }
+        end
+      end
   end
 end