Support wildcard aliases for Drush 8

[greg-1-anderson]

On Pantheon, some organizations have many many sites - so many that the operation to generate the Drush aliases for the org can time out and fail.

For Drush 9, site alias wildcard environments can be used to greatly reduce the number of records that need to be generated, as a single record can represent any number of environments for a single site. For example:

examplesite.site.yml

'*':
  host: appserver.${env-name}.8a5311d2-....pantheon.io
  uri: ${env-name}-a-far-off-site-with-no-org.pantheonsite.io
  user: ${env-name}.8a5311d2-....
  ssh:
    options: '-p 2222 -o "AddressFamily inet"'
    tty: false

If this alias is accessed via drush @examplesite.foobar status, then all occurrences of ${env-name} will be replaced with foobar. If there is, in fact, an environment named foobar, then the alias that is generated on-the-fly will work to access the environment. This technique can be used in any instance where the contents of each sites' environments' alias records are identical save for the environment name.

This technique may be enhanced with an alias-alter hook to give a user-friendly error message if an environment that does not exist is accessed.

This PR adds wildcard alias handling to Drush 8, which will allow this technique to be used in a more uniform way, e.g. on customer's Drupal 7 sites.

A Drush 8 wildcard alias looks like this:

pantheon.aliases.drushrc.php

  $aliases['examplesite.*'] = array(
    'uri' => '${env-name}-a-far-off-site-with-no-org.pantheonsite.io',
    'remote-host' => 'appserver.${env-name}.8a5311d2-....pantheon.io',
    'remote-user' => '${env-name}.8a5311d2-....',
    'ssh-options' => '-p 2222 -o "AddressFamily inet"',
  );

Wildcard aliases also have the advantage of not needing to be regenerated when the user creates a new environment.