Create automatic tasks for each environment in project_settings.host_list.

These can be overridden by creating a function with the same name below,
or in localfab.

Remove the hardcoded ones that don't do anything special.

Fixes #8.
Merge remote-tracking branch 'origin/auto_define_environments' into develop

* origin/auto_define_environments:
  Create automatic tasks for each environment in project_settings.host_list.