Root db access

[martbhell]

Configure a mysql root user.
It is possible to disable it by setting a variable: slurm_manage_mysql_security to False
I would like another eye on this before merging and testing in our test cluster.
Based on travis testing it seems to work when installing from scratch, maybe there are some caveats for when doing it on a running system?

[A1ve5]

Seems legit to me. Must be tested in test system.

[martbhell]

Force pushed because of a rebase with master.

Before the rebase:

• travis shows that installing with this role from scratch still works (and sets mysql root user password)
• on an openstack CentOS7 VM and first using master branch of this role slurm still works, if I then change to this branch then it also still works - and mysql root user has a password.

After rebase:

• travis install also works
• in an openstack VM first configuring from master also works
• in an openstack VM configuringt then the same VM from this branch also works.

Merging this and testing on io.