#66 option for sr_poll to only poll when has_vip.

[psilva]

The sr_poll do_poll script executes on all participants in HA configs (whether or not has_vip is True). In the original use case, the poll script must execute on the backup node to keep the ''ls'' state uptodate on the backup nodes so that only files which haven't been transferred come over when a failover occurs.

in many other (almost all) cases, one really wants to run the poll script only on the active node.
so should add a option to sr_config.py

poll_runs_on_all_nodes (default: False)

and set it to true for the (original) NWS case, but have the default be False.

[psilva]

now: https://github.com/MetPX/sarracenia/issues/29