Feature Request - ee doctor, ee service list and download images on need basis

System Information
- [ ] ee cli info
```
root@example.com:~# ee cli info
+-------------------+--------------------------------------------------------------------------+
| OS | Linux 4.15.0-1035-aws #37-Ubuntu SMP Mon Mar 18 16:15:14 UTC 2019 x86_64 |
| Shell | /bin/bash |
| PHP binary | /usr/bin/php7.2 |
| PHP version | 7.2.17-1+ubuntu18.04.1+deb.sury.org+3 |
| php.ini used | /etc/php/7.2/cli/php.ini |
| EE root dir | phar://ee.phar |
| EE vendor dir | phar://ee.phar/vendor |
| EE phar path | /root |
| EE packages dir | |
| EE global config | /opt/easyengine/config/config.yml |
| EE project config | |
| EE version | 4.0.12 |
+-------------------+--------------------------------------------------------------------------+
```
- [ ] lsb_release -a
```
root@example.com:~# lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description: Ubuntu 18.04.2 LTS
Release: 18.04
Codename: bionic
```
- [ ] docker version
```
root@example.com:~# docker version
Client:
Version: 18.09.5
API version: 1.39
Go version: go1.10.8
Git commit: e8ff056
Built: Thu Apr 11 04:43:57 2019
OS/Arch: linux/amd64
Experimental: false

Server: Docker Engine - Community
Engine:
Version: 18.09.5
API version: 1.39 (minimum version 1.12)
Go version: go1.10.8
Git commit: e8ff056
Built: Thu Apr 11 04:10:53 2019
OS/Arch: linux/amd64
Experimental: false
```
- [ ] docker-compose version
```
root@example.com:~# docker-compose version
docker-compose version 1.24.0, build 0aa59064
docker-py version: 3.7.2
CPython version: 3.6.8
OpenSSL version: OpenSSL 1.1.0j 20 Nov 2018
```

Hi Team, I recently setup EE4 fresh server for one of my personal production site. You guys did excellent job for EE4. Congratualtions :) .

From DevOps point I wanted to add some feature request to EE4. Insteading of creating different issues adding those into single one.

## Feature Request - 1 `ee doctor`
### Purpose:
Verify all the docker images are available or not. If not download them.
### Use Case:
Because of disk space issue I accidenlty deleted on some of the directory from `/var/lib/docker/overlay2/` and so containers refused to start. The solution was to remove all docker images and download them again. I deleted all the images, but now I am not sure which docker images are required for my site. So I created example.com wp site which downloaded all images again. Once all the docker images are downloaded, I deleted the example.com site :) . For this perticular scenario I think `ee doctor` command is useful, which will verify all the docker configuration and iamges is correct or not. If not delete them.

## Feature Request - 2 `ee service list`
### Purpose:
List down all the global services available with EE.
### Use Case:
I failed to find list of available / running global EE service on my server. I think `ee service list` command is useful to get the list of all services.

## Feature Request - 3 Download only required images.
### Use case:
My Server had only 8 GB of disk space and faced disk space issue on server. After anaylysing I found out that EE download some docker images, that I am not using on the server at all. Here is the list of images that EE dowloaded on my server:

```
REPOSITORY TAG IMAGE ID CREATED SIZE
easyengine/php v4.0.2 03e2a331d79c 6 weeks ago 917MB
easyengine/nginx-proxy v4.0.2 e5ba188b1407 7 weeks ago 154MB
easyengine/newrelic-daemon v4.0.0 1682b195a70f 2 months ago 15.9MB
easyengine/php7.0 v4.0.0 f2c622b0aa0d 3 months ago 876MB
easyengine/php7.3 v4.0.0 bdb352d67984 3 months ago 775MB
easyengine/cron v4.0.0 e86e4f2e4e9a 5 months ago 8.83MB
easyengine/postfix v4.0.0 d87174c45b75 5 months ago 475MB
easyengine/mailhog v4.0.0 789b9de98747 5 months ago 19.3MB
easyengine/mariadb v4.0.0 e45653bfaab7 5 months ago 362MB
easyengine/redis v4.0.0 5c605601251d 5 months ago 94.9MB
easyengine/nginx v4.0.0 27f672415832 5 months ago 74.4MB
easyengine/php5.6 v4.0.0 47945e164e8b 5 months ago 841MB
```
I don't use mailhog, newrelic, php5.6, redis on my server. My WordPress site is just simple website without any caching. I think EE should download images based on the requirement only instead of downloading all the images..