improve selection of default release.

set the default release to 'precise' if this is not a known release.
improve the known-release detection by using ubuntu-distro-info if
it is available.  If it is not, fall back to the (now more complete)
static list.
ubuntu-cloud-clone-hook: improve usage
move ubuntu-cloud-clone-hook to ubuntu-cloud-prep

Just rename this, the 'hook' and 'clone' seemed redundant.