Fixed ping plugin under bsd variants.

[AlexanderThaller]

Under version Telegraf 1.9.0 the ping plugin throws the following error under FreeBSD 11.2:

2018-12-05T13:42:50Z E! [inputs.ping]: Error in plugin: host 1.1.1.1: ping: illegal option -- w
usage: ping [-AaDdfnoQqRrv] [-c count] [-G sweepmaxsize] [-g sweepminsize]
            [-h sweepincrsize] [-i wait] [-l preload] [-M mask | time] [-m ttl]
            [-P policy] [-p pattern] [-S src_addr] [-s packetsize] [-t timeout]
            [-W waittime] [-z tos] host
       ping [-AaDdfLnoQqRrv] [-c count] [-I iface] [-i wait] [-l preload]
            [-M mask | time] [-m ttl] [-P policy] [-p pattern] [-S src_addr]
            [-s packetsize] [-T ttl] [-t timeout] [-W waittime]
            [-z tos] mcast-group, exit status 64

This fixes that and changes the flag to -W which was probably intended in that use-case.

Required for all PRs

• Signed CLA.
• Associated README.md updated.
• Has appropriate unit tests.