ssh: add capability to specify a ssh identity file

There are cases when a external ssh identity file needs to be used that
is not added to the running users keychain.

Signed-off-by: Niklas Söderlund <niso@kth.se>
ssh: append args passed to proc ssh to ssh

These arguments where previously ignored but present in the proc ssh
signature.

Signed-off-by: Niklas Söderlund <niso@kth.se>