Support trusty; pre-release support for utopic

- Remove the "prerelease" tag for trusty
- Add preliminary support for utopic
Update PXE install logic

pxe-install-flag was being abused to check both for unattended
installs and cluster installs.  In preparation for debconf support,
define variables for unattended installs (regardless of metapackage)
and set the category variable earlier.

Similarly, "cluster" was being abused to mean "unattended".  This
de-couples the metapackage from the installation method.