s4:dsdb Move module configuration from each ldb into samba_dsdb.c

s4:dsdb Move module configuration from each ldb into samba_dsdb.c

This makes getting the module order correct, the obligation of Samba4
developers, and not system administrators.  In particular, once an ldb
is updated to use only the 'samba_dsdb' module, no further changes to the
ldb should be required when upgrading to later Samba4 versions.

(thanks to metze for the suggestion of samba_dsdb as a long-term
stable name for the module)

Andrew Bartlett