Remove log message on ping timeout

[danielnelson]

If the ping executable returns with exit status 1, don't report an error yet and try to parse output. If the exit status is not 1 return without trying to parse, this is a change from the current behavior.

Here are some output examples:

[[inputs.ping]]
  urls = ["timeout.example.org", "8.8.8.8"]
  count = 1

$ ./telegraf --test
> ping,url=8.8.8.8 packets_transmitted=1i,packets_received=1i,percent_packet_loss=0,minimum_response_ms=14.791,average_response_ms=14.791,maximum_response_ms=14.791 1502758914000000000
> ping,url=timeout.example.org packets_transmitted=1i,packets_received=0i,percent_packet_loss=100 1502758915000000000

$ PATH= ./telegraf --test
2017-08-15T01:02:07Z E! Error in plugin [inputs.ping]: exec: "ping": executable file not found in $PATH
2017-08-15T01:02:07Z E! Error in plugin [inputs.ping]: exec: "ping": executable file not found in $PATH

$ cat ping
#!/bin/sh
echo not the right output

$ PATH=. ./telegraf --test
2017-08-15T00:47:26Z E! Error in plugin [inputs.ping]: Fatal error processing ping output: timeout.example.org
2017-08-15T00:47:26Z E! Error in plugin [inputs.ping]: Fatal error processing ping output: 8.8.8.8

$ chmod -r ping
$ PATH=. ./telegraf --test
2017-08-15T01:02:24Z E! Error in plugin [inputs.ping]: /bin/sh: ping: Permission denied, exit status 126
2017-08-15T01:02:24Z E! Error in plugin [inputs.ping]: /bin/sh: ping: Permission denied, exit status 126

closes #1654

Required for all PRs:

• Has appropriate unit tests.