Add git_refs, git_sha, git_status, and git_last properties to platforms. Load them on provision verify for saving to hostmaster.

- Rename variable to "$options" to reduce confustion between $this->data and $data.
- Create "group" and "environment" properties to represent a drush alias file and environments.
- Write legacy alias files as context name, and drush yaml site aliases with group/environment.
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- Delete all alias files on site delete.

Add site_install_command property and change install task to run an actual process to it won't explode when including a separeate version of drush.

Change install task to run an actual process to it won't explode when including a separeate version of drush.