End support for storing alias files in Vlad's settings directory

[dixhuit]

Currently, the drush_extras role will look for site aliases in Vlad's settings directory before copying them over into the VM. But this role never gets updated when we add or move possible settings directory locations, it also doesn't cater for users who choose not to nest their vlad_settings.yml within a settings directory. All in all, this is unnecessarily fragile.

I'd like to propose that we stop supporting this vlad-specific location for storing alias files and update the drush_extras role to search in the preferred "site-specific" locations for storing site aliases as listed in drush's own example docs ("$ROOT" refers to Drupal site root directory):

• $ROOT/drush/site-aliases
• $ROOT/sites/all/drush/site-aliases
• $ROOT/../drush/site-aliases

This will be a breaking change for anyone choosing to store their site aliases in a Vlad "settings" directory.

[dixhuit]

Uuugh, not enough coffee clearly. The first 2 locations in that list will be automatically "supported" as they're in an existing shared directory:

• $ROOT/drush/site-aliases
• $ROOT/sites/all/drush/site-aliases

I guess we only need add support for the 3rd option that sits outside of the site root:

• $ROOT/../drush/site-aliases

[dixhuit]

OK, the more I mess with this, the more I think we should scrap handling drush aliases (specifically) via Vlad.

Discovering the location of $ROOT/../drush from Ansible is proving trickier than I first thought and this will likely become even trickier as we experiment further with possible locations for Vlad to reside (#253). With this in mind I propose that we remove all site alias handling from Vlad entirely. Users will still be able to add site alias files via the site codebase as mentioned above, plus there's always custom playbooks for more advanced requirements.

I'll get something committed in a minute and update the docs accordingly.