ansible-scylla-monitoring: Move scylla_servers.yml generation to the monitoring role

[igorribeiroduarte]

Currently, scylla_servers generation is done in the node role and this creates an unnecessary dependency from the monitoring role on the node role, since this file is useful only for the former. This patch fixes this by moving the files related to the generation of this file to the monitoring role. It also renames monitoring_config.yml to generate_scylla_servers.yml.

[tarzanek]

@ebenzecri please if you have comments let them still be written down here so @igorribeiroduarte can address thanks guys!