This reverts commit 671cc75.

The authority discovery module was initially enabled by default on validator and sentry nodes with commit bc6e1e7. This change was later on reverted in 671cc75. With this commit the authority discovery module is again enabled by default.

With libp2p/rust-libp2p#1698 the amount of file descriptors needed for the authority discovery Kademlia DHT activities has been drastically reduced. With paritytech/substrate#7018 lookups are spread out instead of done all at once removing any spikes in number of file descriptors, network traffic or CPU usage.

After a testing phase on a couple of our Kusama validator and sentry nodes the authority discovery module is now enabled on the majority of Parity Kusama nodes. For details see either paritytech/substrate#7018 (comment) or consult the corresponding authority discovery Grafana dashboard.

Closes #1544.

Release note suggestion:

The authority discovery module enables Polkadot authorities to discover and directly connect to other authorities. Instead of having to explicitly enable the authority discovery module on validator and sentry nodes, with this release the module is enabled by default. For those not interested in running the authority module please pass the ---disable-authority-discovery flag. As one would expect the --enable-authority-discovery flag is replaced by the the --disable-authority-discovery flag.