Fixed SSH login not working

Fixed SSH login not working

SSHLibrary.Login_With_Public_Key requires the SSH on the
machine where Robot is running not doing key verification and
asking questions of type "Host A.B.C.D was not seen before,
are you sure you want to connect to it?".

This fix uses the newly added "flexible login" keyword which
uses either password if it gets one or public key
authentication if no password provided. This allowed to
bypass these problems with public key authentication by
simply providing a password.

Change-Id: I6c9987ba2871274b4fad75ba56dcadd9623239d5
Signed-off-by: Jozef Behran <jbehran@cisco.com>