Allow performing boot & start operations in groups

Adds top-level configuration options for `group_limit` and `group_wait`.
When a `group_limit` is present, we'll perform app boot & start
operations on no more than `group_limit` hosts at a time, optionally
sleeping for `group_wait` seconds after each batch.

We currently only do this batching on boot & start operations (including
when they are part of a deployment). Other commands, like `app stop` or
`app details` still work on all hosts in parallel.

test/fixtures/deploy_with_group_strategy.yml

@@ -0,0 +1,16 @@
+service: app
+image: dhh/app
+servers:
+  web:
+    - "1.1.1.1"
+    - "1.1.1.2"
+  workers:
+    - "1.1.1.3"
+    - "1.1.1.4"
+
+registry:
+  username: user
+  password: pw
+
+group_limit: 3
+group_wait: 30