Add support for db lib choice

[atcorduneanu]

I used this role to automate the setup of a couple of 389ds servers and I noticed that the database lib is the default,
while I needed mdb for my use case. After reading the documentation page I saw there was no customizable option offered to the user.

I modified the collection inserting a couple of parameters in the v2 inf file template and it worked. I'm currently using it for my setups.
The parameters involved are:

• db_lib
• mdb_max_size

I would happily open a PR for this if anybody is interested as well.

[lvps]

While I personally don't need to customize those parameters in my setup, you're welcome to open a PR and I'll merge it, it may be useful for someone else.