agnhost: allow to disable udp listener on netexec

[aojea]

There are some tests that doesn't need the UDP listener, so they
can disable it.
This is specially needed for tests that use hostNetwork pods, if 2
pods try to bind to the same port, the test will fail because one
of the pod can't be scheduled because of the port conflict.

To keep backwards compatibility, we can add an option to disable
the UDP listener by setting the port number to -1, that is consistent
with the SCTP implementation.

/kind failing-test
/kind flake

NONE

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[aojea]

/assign @claudiubelu @MrHohn
/cc @uablrek

[MrHohn]

/lgtm

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