mongotop does not report activity on secondary

To reproduce, run mongotop --host replset/localhost:27017,localhost:27018 - where localhost:27017 is the primary of the replset replica set while localhost:27018 is a secondary.

Issue: mongotop will only reports activity on the primary even when operations are occurring on the secondary.

Proposal 1: By default, mongotop should allow secondary reads (for the top/server status queries) only if the host is supplied using the replica set format. Furthermore, only one host should be accepted if the format is used - otherwise, it's ambiguous which host should be reported on.

Proposal 2: Same as proposal 1 but without the latter constraint on the number of hosts accepted - mongotop would report on all hosts supplied, or all hosts in the replica set (similar to mongostat's --discover).