Add push_env config

This setting allows you to automatically push env files when deploying.
The default is not to push any files, but you can set it to `all`,
`clear` or `secret` to push the relevant files.

The most useful setting is `clear` which will push the clear env files
every time you deploy.

In addition you can choose the env_type to push when calling
`kamal env push` directly:

```
kamal env push --env-type clear
kamal env push --env-type secret
kamal env push --env-type all # same as kamal env push
```

test/fixtures/deploy_push_clear_env.yml

@@ -0,0 +1,37 @@
+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
+push_env: clear
+
+accessories:
+  mysql:
+    image: mysql:5.7
+    host: 1.1.1.3
+    port: 3306
+    env:
+      clear:
+        MYSQL_ROOT_HOST: '%'
+      secret:
+        - MYSQL_ROOT_PASSWORD
+    files:
+      - test/fixtures/files/my.cnf:/etc/mysql/my.cnf
+    directories:
+      - data:/var/lib/mysql
+  redis:
+    image: redis:latest
+    roles:
+      - web
+    port: 6379
+    directories:
+      - data:/data
+
+readiness_delay: 0