schema: Add option of specifying the base schema for a provision

schema: Add option of specifying the base schema for a provision

Add the ability to override the base schema files being used for the
new provision, e.g. instead of using the default supported schema,
the code can now potentially specify an older or newer schema to use.

Signed-off-by: Tim Beale <timbeale@catalyst.net.nz>
Reviewed-by: Garming Sam <garming@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>