File contention with multiple SSH sessions and --gen-key-pair

[klauern]

Related to #175, we also have some contention with settings that use the --gen-key-pair flag and a path:

Match User ubuntu* Host i-*
    IdentityFile ~/.sigil/%h/temp_key
    IdentitiesOnly yes
    ForwardAgent yes
    ProxyCommand sh -c 'sigil ssh --port %p --gen-key-pair --gen-key-dir "${HOME}"/.sigil/%h/ --os-user %r %h'

In connecting to multiple hosts at once, similar to Chef, Ansible, tmux-cssh, et al, we get connection errors:

no such identity: /Users/me/.sigil/i-11111111111111111/temp_key: No such file or directory

I'm not sure how to configure around this, as the ~/.ssh/config uses a static temp_key path, and I would need this file to be unique every time sigil is run.