s4:provision Allow a specific prefix map to be loaded into a new schema provision

s4:provision Allow a specific prefix map to be loaded into a new schema provision

This allows the prefixMap from a DRS server to be used when loading
the schema from the local files.  This helps us then import other
schema with this map in place.

Andrew Bartlett

Signed-off-by: Kamen Mazdrashki <kamenim@samba.org>