Add timeout option to IPMI sensor input plugin

[m4ce]

Solves #2817.

Required for all PRs:

• CHANGELOG.md updated (we recommend not updating this until the PR has been approved by a maintainer)
• Sign CLA (if not already signed)
• README.md updated (if adding a new plugin)

[danielnelson]

Looks good, can you add it to the CHANGELOG?

[m4ce]

@danielnelson, I've updated the CHANGELOG as requested.

[danielnelson]

Can you change the default to 5s as well? Since the default interval is 10s we should keep it under that.

[m4ce]

The problem with 5s is that is just too low and users would have to often override that. If you are okay with it, I will change it back to 5s.

[danielnelson]

closes #1565

[danielnelson]

How much time do we need to query a single localhost server? If we only need >5s for mutliple remote servers than I am comfortable requiring the user to increase the timeout.

[m4ce]

So, via the IPMI kernel interface (which is local), it takes more than 5 seconds on some of my boxes.

# time ipmitool sdr >/dev/null

real    0m8.026s
user    0m0.000s
sys     0m0.003s

I'd imagine over the network should be more expensive.

[danielnelson]

So maybe we should set the timeout = 20s and also add interval = 30s, similar to what we did for the cloudwatch input.

[m4ce]

@danielnelson - I've made the changes as requested. Let me know if there's anything that needs fixing.