Fail helpfully if uuidgen is missing

If uuidgen is not installed we currently fail when performing tune2fs.

Change-Id: I4062eb1d62322c455dac5b83b856f454da211320
Allow absolute path to image with ironic-agent

The element was prepending the work directory forcibly, which will have
problems when the image name has an absolute path, which is allowed.

Change-Id: I0cb7b96e24daab8ee73611936af72074c70ac1aa
Closes-Bug: #1400405
Merge "Allow absolute path to image with ironic-agent"
Merge "Fail helpfully if uuidgen is missing"