Add the ability to manually execute checks against an application

Checks can be run in several ways:

- checks:run APP
- checks:run APP PROCESS_TYPE
- checks:run APP PROCESS_TYPE.CONTAINER_INDEX

This will reuse the existing checks infrastructure as defined by the checks plugin, and therefore if no checks file is defined, it will default to the normal Dokku container check.

Closes #2218

plugins/common/functions

@@ -624,7 +624,7 @@ dokku_deploy_cmd() {
       trap 'kill_new $id' INT TERM EXIT
       if [[ "$(is_app_proctype_checks_disabled "$APP" "$PROC_TYPE")" == "false" ]]; then
         dokku_log_info1 "Attempting pre-flight checks"
-        plugn trigger check-deploy  "$APP" "$id" "$PROC_TYPE" "$port" "$ipaddr"
+        plugn trigger check-deploy "$APP" "$id" "$PROC_TYPE" "$port" "$ipaddr"
       fi
       trap -        INT TERM EXIT