[keyserver] move isPrimaryNode to keyserver utils

Summary: Moves isPrimaryNode into utils in anticipation of using it in determining whether to run cron jobs

Test Plan: exported COMM_NODE_ROLE and console logged in primary only code as primary and secondary. Confirmed expected behavior

Reviewers: ashoat, varun

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D12846