Fixed "close connection" action in MDSAL suite

Fixed "close connection" action in MDSAL suite

Sending "^D" to the netconf connection does not guarantee
that the connection is closed properly, leading to a
spurious test failure due to failed teardown.

The proper way is to open two connections, one "control" and
one "netconf". The "netconf" connection then connects to
netconf as usual and the "control" connection is used to
control the SSH process connecting to netconf. It is used to
obtain the PID of the "sshpass" process which governs the
connection and to send the kill signal to that process
once the connection is to be closed.

Change-Id: I3a0bc1e03d1af9619ced3fa7dfcd45aad3d696fa
Signed-off-by: Jozef Behran <jbehran@cisco.com>