dump_settings: gather remote host info before dumping

dump_settings fails to write settings file when any of the
variables are not initialized like the one below.
    CONFIG_COMPUTE_HOSTS: '{{ hostvars[compute_name].private_ip }}

Running get_nodes should solve most of the foreseeable issues and
would only be run when 'dev' tags is specified. To dump_settings
without getting_nodes, use - 'dump_settings' tag.

Change-Id: Ifcc122f5e7a722787faaf90e99bea8ad01b77862