Add support for specifying Git ref on required repositories (#26)

resource-dispatcher/documentation/configuration/repositories.md

@@ -12,8 +12,11 @@ repositories:
       ssh_private_key: /my/path/to/an/ssh/key
   - name: automation-repo-2
     url: https://github.com/my-org/some-ansible-playbooks-2.git
+    ref: integration
 ```
 
+Both `ref` and `secret.ssh_private_key` are optional, to be used when a Git ref other than `master` is desired, or an SSH pull secret is required, respectively.
+
 Upon application startup, these repositories are cloned. They can be referenced from that point forward by their name as defined here, _not_ by any part of their Git URL.
 
 **The same scheduling mechanism used to trigger tasks is _automatically configured_ to pull new changes from these repositories every two minutes, so updates to these dependencies should be picked up _without any user intervention required_.**

resource-dispatcher/repositories/__init__.py

@@ -17,8 +17,9 @@ def configure_repository(repository):
 
     if not os.path.exists(repo_directory):
         os.makedirs(repo_directory)
+        ref = repository["ref"] if "ref" in repository else "master"
         print(f"Cloning {repository['url']}...")
-        repo = Repo.clone_from(repository["url"], repo_directory, branch="master", env=env)
+        repo = Repo.clone_from(repository["url"], repo_directory, branch=ref, env=env)
         print(f"Cloned {repository['url']}")
     else:
         repo = Repo.init(repo_directory)