Refactor use deployment command

* Renamed 'deployment' filename in 'globals' in order to
use this file with other use subcommands such as 'task'
* Refactored by moving code into new generic functions that
  * check if the 'use_command' file exists:
  _ensure_rally_configuration_dir_exists(self)
  * update env var in the 'use_comand' file:
  _update_attribute_in_use_file(self, attribute, value)

Change-Id: I7734d34f99b468001bd431f98d97e4dade2236cd