It allows us to have more clear and controllable unit tests and
to move towards bringing all file operations in airshipctl to a
common standard for all source files (using only document
filesystem module).
Change-Id: I560edb7c8e9cd682f3c188527245fcd40045e16c
Signed-off-by: Ruslan Aliev <raliev@mirantis.com>
Relates-To: #415