Add shell probe

[AndrienkoAleksandr]

We should create "shell probe". For example if container doesn't has shell "/bin/bash" we can try to create new one exec with "/bin/sh" and so on... We can have list of such kind of "probes".

[benoitf]

maybe check as well /etc/passwd that defines shell per users

[AndrienkoAleksandr]

We should get info "about shell" from predefined places ("/etc/passwd"), if we have no info about shell in container we can guess to prevent fail terminal creation.