Improve mkimage-debian script to also tag using the release version number of the final image (6.0.7, 7.0, etc.)

This is as discussed on #447.
Use default mirror from debootstrap when not explicitly provided, and add better target directory naming
Merge pull request #484 from tianon/mkimage-debian

* Contrib: updated mkimage-debian
Update the crashTest to have the dockerpath in env
add network endpoint for crashTest