Allow a different SSH host to be set in Diffusion

Summary:
Ref T6941. In the cluster (and in other reasonable setups) we've separated SSH load balancers from HTTP load balancers.

In particular, ELBs will not let you load balance port 22, so this is likely a reasonable/common issue in larger clusters in AWS.

Allow users to specify an alternate host for SSH traffic.

Test Plan: Set host to someting different, saw it reflected in UI.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T6941

Differential Revision: https://secure.phabricator.com/D11800