Minimise holding the deploy lock

If we get an error we'll only hold the deploy lock if it occurs while
trying to switch the running containers.

We'll also move tagging the latest image from when the image is pulled
to just before the container switch. This ensures that earlier errors
don't leave the hosts with an updated latest tag while still running the
older version.

lib/mrsk/commander.rb

@@ -2,11 +2,12 @@ require "active_support/core_ext/enumerable"
 require "active_support/core_ext/module/delegation"
 
 class Mrsk::Commander
-  attr_accessor :verbosity, :lock_count
+  attr_accessor :verbosity, :holding_lock, :hold_lock_on_error
 
   def initialize
     self.verbosity = :info
-    self.lock_count = 0
+    self.holding_lock = false
+    self.hold_lock_on_error = false
   end
 
   def config
@@ -115,6 +116,14 @@ class Mrsk::Commander
     SSHKit.config.output_verbosity = old_level
   end
 
+  def holding_lock?
+    self.holding_lock
+  end
+
+  def hold_lock_on_error?
+    self.hold_lock_on_error
+  end
+
   private
     # Lazy setup of SSHKit
     def configure_sshkit_with(config)