Partially revert "Differentiate ZeroMQ from RAET based installations."

This partially reverts commit e4179cdc68ebced43297c8b0a33f769a418ec054.
Move the `--salt-transport` to the install command
Update documentation accordingly
Start off with the common salt install requires
Remove ending `install`
Support the best possible option for all

* pip based installations default to ZeroMQ:

  pip install salt

* pip based installations can additionally install REAT deps

  pip install salt[RAET]

On local checkout's:

* pip based installation with requirements files can choose which, or both, just use the right requirements file(s)

  pip install -r raet-requirements.txt

* pass ``--salt-transport` to choose which, `zeromq`, `raet` or `both`. Choosing `raet` removes the `zeromq` deps
Allow transport to be none to skip ZeroMQ deps
Merge pull request #13526 from s0undt3ch/hotfix/setup-raet

simpler requirements setup