Load ssh keys into the ssh-agent before vagrant up

[becw]

Some of our environments run ssh commands (e.g. git clone with an ssh path) that are reliant on key forwarding. When the host doesn't have any keys loaded (check ssh-add -l), no keys are forwarded to the VM and these operations fail -- whereas if they were run on the host, the keys would be transparently loaded at that point.

Could we use a trigger to run ssh-add on vagrant up? And maybe only run it if ssh-add -l is empty?