Being able to discover DIB's version from the command itself is
convenient. This patch adds a --version option to the disk-image-create
command, failing gracefully if diskimage-builder is not installed.
This adds an explicit dependency on pbr to the requirements since this
is required to run diskimage_builder/version.py outside of a test
environment.
This patch consciously chooses to only provide the long-form option
and no '-v' to allow for the future possibility that a '-v' might
indicate '--verbose' in the future.
Change-Id: I9fc084774d6c7a39a944b07680b3eb8be8e34f9c
76a8d3704b