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/integration/docker/deployer/app/config/deploy.yml

@@ -33,3 +33,4 @@ accessories:
     roles:
       - web
 stop_wait_time: 1
+push_env: clear