Adds optional support for lmdb choice and sizing

[atcorduneanu]

This pr adds dirsrv_use_mdb and dirsrv_mdb_max_size.
Both those parameters modify the installation template only if explicitly set, as I think that the correct behaviour is to rely on the default values of the 389ds_base parameters that those options control. I.e. if the user does not have a choice, they should go with the default value of the distribution's 389ds_base, and not have a default value enforced by this role.

-- from https://www.port389.org/docs/389ds/FAQ/Berkeley-DB-deprecation.html

Starting from version 3.0.0 New instances are no longer created with BerkeleyDB by default but with LMDB.

Newly created instances are still created with BerkeleyDB by default while libdb is flagged as deprecated since Fedora 33, this change is about to:
 + create LMDB instances by default.
 + warn that BerkeleyDB is deprecated and may be removed in future versions.

[atcorduneanu]

#72

[lvps]

Thanks, merged it. The CI is still failing due to an unrelated problem (#68) but your changes look good to me