Use encoded string of mysql password for slurm_mysql_password

[martbhell]

The "encrypted" parameter can be used instead of "password" to the mysql_user module since ansible 2.0

http://docs.ansible.com/ansible/mysql_user_module.html

As the slurm_mysql_password variable is set manually by the users of this role, probably to implement this we should create a new variable (perhaps slurm_mysql_encrypted_password) and use that if it's set and if not use the slurm_mysql_password.

[martbhell]

I wonder if the password still needs to be written in clear text to the slurm config file. Some testing of that is needed.