Adding an option to control the DIB repo branch for sahara-image-elements

Currently the diskimage-create.sh script in Sahara-Image-Elements always uses the head of the master branch in diskimage-builder. This blueprint proposes adding a command line switch, and potentially an environment variable, to change this behavior.

By default, without the command line switch, diskimage-create.sh will use a predfined tag for the diskimage-builder repository clone. If the user specifies the command line switch, diskimage-create.sh will use the head of the master branch in diskimage-builder. Optionally the user could specify an evironment variable to control the exact diskimage-builder branch.

The command line switch is proposed as "-m" for master.

The environment variable is proposed as "DIB_REPO_BRANCH".