Add parameter to set my.cnf group

This apply to the system /etc/mysql/my.cnf not the personnal ".my.cnf" files.
Default to root, as before.
Add an unmanaged_password parameter

In the case we don't want to have a default password.
refactor unmanaged_password code with error checking
Merge pull request #64 from ckaenzig/add_missing_parameters

Add missing parameters
add mycnf_mode parameter to fine-tune my.cnf file