exec plugin: allow using glob pattern in command list

[peekeri]

Allow using glob pattern in the command list in configuration. This enables for
example placing all commands in a single directory and using /path/to/dir/*.sh
as one of the commands to run all shell scripts in that directory.

Glob patterns are applied on every run of the commands, so matching commands can
be added without restarting telegraf.

This is a second try for this change. The previous PR #1127 broke existing functionality in two ways:

• commands with arguments got ignored
• commands in PATH got ignored

These issues are addressed now by:

• splitting the command into actual command and arguments before globbing
• using the command as-is in case glob returns no matches.

Added also a couple of tests to see that those are actually fixed. Not sure though if we can assume presence of /bin/echo?

[sparrc]

@peekeri sorry about that, we had a problem with our CI script, please rebase and try pushing again

[peekeri]

@sparrc all tests passed now. Is it ok to assume that /bin/echo is present when running the tests? Tests passed at least, so I guess from that point of view it is ok, but do the tests have to pass for example on Windows machine?

[sparrc]

please limit line lengths to 80 characters, especially for the config file.

Also please rebase and update the changelog again so that your change falls into the 0.13.1 release.