Add onidle chain element into cmd-nse-icmp-responder and cover it by docker tests

[d-uzlov]

Overview

We need to test the new onidle chain element.

Implementation details

1. Add onidle element into responder endpoint chain, and configure it to stop the server on callback.
2. Add timeout parameter to server config.
   Timeout should be defined as a pointer in the config to allow to determine whether the corresponding environment variable is set. If env variable was not set, then we shouldn't add the element.
3. Add a test that checks that the server successfully dies on specified timeout.

References

networkservicemesh/sdk#892

[edwarnicke]

It seems like the simplest way here is to simply have onidle.NewServer interpret a timeout of 0 as 'no timeout' and return null.NewServer ... that way we can set an NSM_ONIDLE_TIMEOUT env variable with default 0 and things work as normal unless its set, and if it is set, we get the onidle behavior. Thoughts?