Allow omit_containers["db"] for projects

[rfay]

The Problem/Issue/Bug:

#1490 requested the ability to not create the db (mariadb) container in cases where it's not used.

People want this when they're using sqlite, or postgres, or another custom database setup.

How this PR Solves The Problem:

• Allow omit_containers["db"] at the project level (not global)
• If omit_containers["db"] the dba container is also omitted because it's useless
• Remove most commands that access the db in the case it's omitted (like export_db)
• Update ddev describe to leave stuff out.

Manual Testing Instructions:

• Use omit_containers["db"] and see how it goes
• Make sure things work without it

Automated Testing Overview:

No changes at this point

Related Issue Link(s):

Release/Deployment notes:

[rfay]

Artifacts for this are at https://circleci.com/gh/drud/ddev/17589#artifacts/containers/0

If using macOS or linux homebrew you can download the macOS tarball, then brew unlink ddev and untar the tarball, cp ddev /usr/local/bin. When done, you can rm /usr/local/bin/ddev && brew link ddev again. For windows, just use the installer.