Bug 8989 - Create just one DS for each test-tool's simulated netconf device

Bug 8989 - Create just one DS for each test-tool's simulated netconf device

When using netconf test-tool with option --md-sal set to true, we expect
that each test-tool's simulated netconf device will have its own md-sal
persistent datastore. However, current implementation creates own
datastore for each netconf session.

Map newly created simulated device's datastore to its server socket
instead of netconf session's socket.

Change-Id: I2d5a0295c8cbff6d29baafa36bdbbe6aba8119c6
Signed-off-by: Jakub Morvay <jmorvay@cisco.com>
(cherry picked from commit c65ab49a8a2f72446e13e0bb0f8395722c455c6b)