add procval plugin

[zbindenren]

We had an issue in our company, that we had not enough entropy on the system for our current ssl ciphers. The problem especially occurred on virtual systems.

So I wrote a small plugin which reads integer values from defined proc files and sends them to inlfuxdb. With that plugin it is possible to monitor system entropy.

[sparrc]

thanks for the contribution @zbindenren, but I'm going close this because it seems a little "one-off" for having as a dedicated plugin in Telegraf. It would be very simple to write up a shell script to get this info and use the exec plugin to parse the data.

There will also soon be a way to parse just a single value from an executable (see #849), that way, you could just setup the exec plugin like this:

[[inputs.exec]]
  name_override = "entropy"
  commands = ["cat /proc/path/to/entropy"]
  data_format = "value"
  data_type = "integer"

which will allow you to get the entropy without having to write a separate script