DiscoverySeeds: hard coded instances to be discovered upon startup

[shlomi-noach]

Introducing a new configuration variable, DiscoverySeeds, an array of instances, as follows:

  "DiscoverySeeds": [
    "host1:3306",
    "host2",
    "host3:3307"
  ],

This config allows for a hard-coded seeding of a list of hosts. This is useful for bootstrapping a new setup of orchestrator where the identity of MySQL hosts is known up front. This removes the initial need for hosts to present themselves to orchestrator and/or for automation to ask orchestrator to discover said hosts.

Seeded hosts are discovered even if instructed to be forgotten.

[shlomi-noach]

Works as expected