Define the ssh identity file through supfile.yml or an environment variable

[leeif]

• A need for me to use sup in the CI like Jenkins and it's not a good idea to set the ssh agent every time when the job is executed. The current code only read the identity file which name is id_*. Why not make it to be definable ?

• Error when parsing the identity file in the openssh private key format. This can be solved by updating the version of crypto/ssh package.

The related PR #166