Add a pre-connect hook

This can be used for hooks that should run before connecting to remote
hosts. An example use case is pre-warming DNS.

test/cli/main_test.rb

@@ -23,6 +23,7 @@ class CliMainTest < CliTestCase
     Mrsk::Commands::Hook.any_instance.stubs(:hook_exists?).returns(true)
 
     run_command("deploy").tap do |output|
+      assert_match /Running the pre-connect hook.../, output
       assert_match /Log into image registry/, output
       assert_match /Build and push app image/, output
       assert_match /Ensure Traefik is running/, output

test/integration/docker/deployer/app/.mrsk/hooks/pre-connect

@@ -0,0 +1,8 @@
+#!/bin/sh
+
+echo "About to lock..."
+if [ "$MRSK_HOSTS" != "vm1,vm2" ]; then
+  echo "Expected hosts to be 'vm1,vm2', got $MRSK_HOSTS"
+  exit 1
+fi
+mkdir -p /tmp/${TEST_ID} && touch /tmp/${TEST_ID}/pre-connect

test/integration/main_test.rb

@@ -8,16 +8,16 @@ class MainTest < IntegrationTest
 
     mrsk :deploy
     assert_app_is_up version: first_version
-    assert_hooks_ran "pre-build", "post-deploy"
+    assert_hooks_ran "pre-connect", "pre-build", "post-deploy"
 
     second_version = update_app_rev
 
     mrsk :redeploy
     assert_app_is_up version: second_version
-    assert_hooks_ran "pre-build", "post-deploy"
+    assert_hooks_ran "pre-connect", "pre-build", "post-deploy"
 
     mrsk :rollback, first_version
-    assert_hooks_ran "post-deploy"
+    assert_hooks_ran "pre-connect", "post-deploy"
     assert_app_is_up version: first_version
 
     details = mrsk :details, capture: true