[JENKINS-33596] disable UDP broadcast & jmDNS by default, make configurable

[kzantow]

Moves configuration for jmDNS and UDP broadcast from Jenkins constructor to global security configuration items.

Addresses: https://issues.jenkins-ci.org/browse/JENKINS-33596

TODO:

• Ensure upgrading from prior versions results in these services being enabled as they were

[daniel-beck]

Would it make sense to introduce a network-discovery plugin, that bundles these features and possibly a fixed version #1301?

CC @jglick who IIRC said in the past that UDP broadcast and DNS multicast should be a new plugin.

[daniel-beck]

FWIW I think even if it's a separate plugin, having UI options for this is useful. So these changes (detach + configurable) are complementary.

[kzantow]

@daniel-beck right, I was taking a piecemeal approach to this... side note: I don't want to be the maintainer of the network-discovery plugin :)

[amuniz]

But the fields were removed from Jenkins, so I guess this information is stored now in its own XML file, no?