galera_server extra repository simplification

[Jean-Philippe Evrard]

galera_server role is using a lot of code that could be heavily simplified:
- CentOS/Ubuntu are splitted but do basically the same
- We can use an ubuntu/centos package to do the repo install of percona tools

I think all of these two code path could be merged in one, by using the ansible package: module with one variable that will be set in the two platforms (two different package urls for adding the repo).

[Jonathan Rosser (jrosser)]

We would need different module parameters to be passed to the apt/dnf variants of calling the package module so i don't think that this overall simplifies things.