Host specific env with tags

Allow hosts to be tagged so we can have host specific env variables.

We might want host specific env variables for things like datacenter
specific tags or testing GC settings on a specific host.

Right now you either need to set up a separate role, or have the app
be host aware.

Now you can define tag env variables and assign those to hosts.

For example:
```
servers:
  - 1.1.1.1
  - 1.1.1.2: tag1
  - 1.1.1.2: tag2
  - 1.1.1.3: [ tag1, tag2 ]
env_tags:
  tag1:
    ENV1: value1
  tag2:
    ENV2: value2
```

The tag env supports the full env format, allowing you to set secret and
clear values.

test/fixtures/deploy_with_env_tags.yml

@@ -0,0 +1,29 @@
+service: app
+image: dhh/app
+servers:
+  web:
+    - 1.1.1.1: site1
+    - 1.1.1.2: [ site1 experimental ]
+    - 1.2.1.1: site2
+    - 1.2.1.2: site2
+  workers:
+    - 1.1.1.3: site1
+    - 1.1.1.4: site1
+    - 1.2.1.3: site2
+    - 1.2.1.4: [ site2 experimental ]
+env:
+  clear:
+    TEST: "root"
+    EXPERIMENT: "disabled"
+env_tags:
+  site1:
+    SITE: site1
+  site2:
+    SITE: site2
+  experimental:
+    env:
+      EXPERIMENT: "enabled"
+
+registry:
+  username: user
+  password: pw