Build subtree DN when using it instead of globally

This fixes the bug where only the BASE_DN gets reset in the caller: In
this case the VIRTUAL_MACHINES_DN is already set using the default
BASE_DN (dc=foss-cloud,dc=org) and searches will therefore fail.

Renaming VIRTUAL_MACHINES_DN to VIRTUAL_MACHINES_SUBTREE to reflect that
it is not a complete DN anymore but only the prefix to the BASE_DN.