Improve router-compose and ssh-auth-compose handling, fixes #1779, fixes #2476

[rfay]

The Problem/Issue/Bug:

1. Doing a ddev stop or ddev restart when you've never done a ddev start to create the router-compose.yaml causes a fatal error (Unable to find .ddev/router-compose.yaml when restarting ddev when it has never been started #1779)
2. It's never been possible to override the router and ssh-agent configs, which makes development hard sometimes, and provides no workarounds for user needs.
3. Letsencrypt volume (recent addition) was being created even if letsencrypt was not in use.

How this PR Solves The Problem:

• Create the .ddev-router-compose.yaml before stop
• Allow overriding with router-compose..yaml and ssh-auth-compose..yaml
• Make sure the letsencrypt volume doesn't get created by default

Manual Testing Instructions:

• ddev poweroff && rm ~/.ddev/.ddev-router-compose* && ddev stop. It should work fine.
• Add new ssh-auth-compose..yaml and router-compose..yaml like this gist and verify that when the container gets created (after ddev poweroff the expected stuff is used (docker exec ddev-router bash -c 'echo $ANSWER' should get "42", as should docker exec ddev-ssh-agent bash -c 'echo $ANSWER'
• docker volume rm ddev-router-letsencrypt - failure is OK. Then ddev start and verify that no letsencrypt volume is created.

Automated Testing Overview:

Related Issue Link(s):

Release/Deployment notes:

This changes the names of the router-compose and ssh-auth-compose files to hidden files, .router-compose.yaml, .router-compose-full.yaml, .ssh-auth-compose.yaml, .ssh-auth-compose-full.yaml