Fix mysql-server ignoring root_user_use_databag attribute.

Addresses: bug #1273353

The mysql-server recipe was not checking the
default['openstack']['db']['root_user_use_databag'] attribute to see if
it should use the node['mysql']['server_root_password'] attribute or
check a databag for the mysql root password.

Change-Id: I589f7e43fc1f15c072d3e50dab6cef25f367075b