Add ssh-agent app

[ip1981]

Add ssh-agent app (multiple instances). This app should support options for UNIX socket, user to run as (the socket owner), and list of private SSH keys (and pass-phrases?).

When deployed, the UNIX socket is exposed to other applications via the SSH_AUTH_SOCK environment variable. Possible consumers: Jenkins.

Note, that Jenkins has the ssh-agent plugin, which does exactly the same, but it has some bugs, and separate ssh-agent application is more general solution.