[keyserver] configure separation of primary secondary logic

Summary:
This introduces COMM_NODE_ROLE which specifies whether a node is a primary or secondary node.

Certain logic in keyserver/src/keyserver.js should only be performed by a primary node. This includes:

• run the migration
• establish tunnelbroker connection
• print the QR Code
• create authoritative keyserver config files

Both secondary and primary nodes should still perform:

• have the master process create fork processes
• route express traffic
• verify the user is logged in

Depends on D12691

Test Plan:
Confirmed through logs that secondary nodes didn't establish tunnelbroker connection.
Will test further if migration is primary only later in stack with migration script

Reviewers: ashoat, varun

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D12708