Ddev: Use DRUSH_OPTIONS_URI instead of creating a drush.yml file

[jcnventura]

Is your feature request related to a problem? Please describe.
ddev is currently having to create a drush.yml file to store the site's primary URL. This can conflict with existing files.

Describe the solution you'd like
Drush 9 and above support setting this via an environment variable called DRUSH_OPTIONS_URI. We should remove the creation of the ddev-provided drush.yml file (keeping for now the drushrc.php for Drush 8 installs, as this solution is not applicable there).

Describe alternatives you've considered
We can also keep creating the existing file. With all the side-effects and collisions that it has.

Additional context
Related to #2196

[rfay]

There's another fantastic reason for doing this: When people use a repo that has been used for ddev in a different context (lando or something), the drush.yml can cause trouble. So using DRUSH_OPTIONS_URI would limit that damage.

[pirog]

@rfay FWIW DRUSH_OPTIONS_URI is what we recommend and use in Lando for similar "limit the damage" reasons.

That said, i do feel like there may be some marketing benefits to DDEV if Lando or other tools users ran lando drush uli and got a ddev branded URL in return ;)

WAIT I THOUGHT I WAS USING LANDO?
AM I USING DDEV?
SHOULD I USE DDEV?
I DONT KNOW WHAT TO BELIEVE ANYMORE!
_WE ARE ALL LIVING IN A GIANT COMPUTER SIMULATION_

[rfay]

@jcnventura and @truls1502 I hope you can test the prerelease v1.15-alpha5 for the DRUSH_OPTIONS_URI support while it's still in prerelease, thanks!