Geo: Make adding a Secondary node easier

Problem

Adding a secondary GeoNode record is a manual, error-prone process. In particular, the name field must match exactly.

The following discussion from !745 (merged) should be addressed:

• @ashmckenzie started a discussion:

There isn't a convenient rake task to add the secondary node because the command must be run on the secondary, but written on the primary.

Proposal

First iteration

• Add rake gitlab:geo:announce_secondary to be run on a new secondary that securely announces it to the primary node.

Then choose one:

• Most convenient and least work option: The primary automatically adds a GeoNode record for the secondary (if this can done safely).
• Otherwise: Click "Add this secondary" in the primary Geo Nodes UI.

Second iteration

• A new secondary basically runs rake gitlab:geo:announce_secondary every minute if a primary GeoNode exists and the machine doesn't match any GeoNode.