Update dig.py error handling

[punkrokk]

I have added an exception to the linux.dig action script to handle the situation where the dig command is not installed, which, it seems on CentOS/RHEL gets the user this error:

{
  "stdout": "",
  "result": "None",
  "stderr": "Traceback (most recent call last):
  File \"/opt/stackstorm/st2/lib/python2.7/site-packages/python_runner/python_action_wrapper.py\", line 334, in <module>
    obj.run()
  File \"/opt/stackstorm/st2/lib/python2.7/site-packages/python_runner/python_action_wrapper.py\", line 193, in run
    output = action.run(**self._parameters)
  File \"/opt/stackstorm/packs/linux/actions/dig.py\", line 30, in run
    stdout=subprocess.PIPE)
  File \"/usr/lib64/python2.7/subprocess.py\", line 711, in __init__
    errread, errwrite)
  File \"/usr/lib64/python2.7/subprocess.py\", line 1327, in _execute_child
    raise child_exception
OSError: [Errno 2] No such file or directory
",
  "exit_code": 1
}

This doesn't seem to be an issue on Ubuntu. It appears the dnsutils is installed. At least on the vagrant st2 dev environment. As you can see there is now a better error message if we get an OSError exception, suggesting that dig is either not on the PATH or installed.

I have also update the linux pack readme to include the dig action.

Since this is a core pack and installed by default, I opened the below PR to install bind-utils by default in EL7 distros.
StackStorm/st2-packages#619

[punkrokk]

@blag updated based on your requests.

[arm4b]

Will wait for @blag for his review,

but a small nitpick from me: I think this improvement will also need a CHANGELOG record since it's part of st2 core.

[blag]

One small wording change, and add a changelog please.

Other than that, this looks good to go! 👍

[blag]

Thanks for the contribution!