Add 'purpose', 'physicalHost' to debianServer

Patch from HE: Add a purpose and a physicalHost attribute to the
debianServer objectClass.  Purpose is used to store the task of
the machine, like buildd, or porterbox or similar.  phyiscalHost
is for setups with virtualisation, where one host runs on top of
another one.  This information can then also be used by nagios and
friends.