tmux config should persist ssh key across all panes

[sethboyles]

see here: https://werat.github.io/2017/02/04/tmux-ssh-agent-forwarding

@JenGoldstrich

[cf-gitbot]

We have created an issue in Pivotal Tracker to manage this:

https://www.pivotaltracker.com/story/show/177351615

The labels on this github issue will be updated when the story is started.