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

test/integration/main_test.rb

@@ -7,6 +7,7 @@ class MainTest < IntegrationTest
     assert_remote_env_file "CLEAR_TOKEN=4321", :clear
     assert_remote_env_file "SECRET_TOKEN=1234", :secret
     remove_local_env_file
+    remove_remote_env_file :clear
 
     first_version = latest_app_version
 
@@ -15,6 +16,7 @@ class MainTest < IntegrationTest
     kamal :deploy
     assert_app_is_up version: first_version
     assert_hooks_ran "pre-connect", "pre-build", "pre-deploy", "post-deploy"
+    assert_remote_env_file "CLEAR_TOKEN=4321", :clear
 
     second_version = update_app_rev
 
@@ -70,6 +72,10 @@ class MainTest < IntegrationTest
       deployer_exec("rm .env")
     end
 
+    def remove_remote_env_file(env_type)
+      docker_compose("exec vm1 cat /root/.kamal/env/roles/app-web-#{env_type}.env")
+    end
+
     def assert_remote_env_file(contents, env_type)
       assert_equal contents, docker_compose("exec vm1 cat /root/.kamal/env/roles/app-web-#{env_type}.env", capture: true)
     end