Fix: ping resource: Use fping6 for IPv6 hosts

[Werkov]

ping RA doesn't work correctly with IPv6 addresses, it should use fping6instead of fping. I've implemented very simple address parsing that is not as precise as that in resource agents, however, it makes do only with shell builtins.

Cc: @gao-yan

[kgaillot]

It's better than what we had, anyway.

We should probably do the same for ping/ping6.

A more general solution might be to add a host6_list parameter, or to dynamically separate host_list into "words with :" and "words without :".

[jnpkrn]

To be noted that ping utility from iputils (finally!) converged on a single CLI interface:
iputils/iputils#5

We can only hope fping will follow this example for the overall sanity.

[jnpkrn]

...see also: schweikert/fping#80

For the record, Fedora brings transparent IPv6 support in "ping"
command as of release 24:
https://fedoraproject.org/wiki/Changes/PingIpv6#Ping_IPv6

[kgaillot]

Those are all good news, though they make it a bit more complicated for us, since we'll have to figure out what version is installed on the local system and call it appropriately