Use single mustache template to generate docker-compose.yml

[kirtangajjar]

Currently both site-command and site-wp-command has a seperate docker-compose.yml. This makes making changes and keeping them consistent harder.

We need to keep only single docker-compose.yml in any one repo (preferably easyengine as we already need it for global docker-compose.yml). Whenever we need to generate docker-compose.yml, we'll use that template.

[mrrobot47]

Comments from this PR discussion are relevant for this. Similar idea, to reduce the repeatability has been discussed here. To add the mustache template in the core as well as creating docker-compose utility to further reduce the steps in the site-commands.