Make import/export use the auxiliary ssh library to build the remote commands.

This avoids forgetting some parameters, as it's happening right now 
(the correct known host file is not being passed)

In order to do so we split SSHCall into an auxiliary BuildSSHCmd which builds
the command but doesn't actually call it, and SSHCall itself which runs RunCmd
on top of BuildSSHCmd's result. BuildSSHCmd is then explicitely called by 
import/export who has to build a more complex command to be run later.