Decouple netconf-ready suite and netconf connector.

Decouple netconf-ready suite and netconf connector.

Netconf connector is going out of favor with Beryllium
implementing the "topology way" of Netconf configuration [0]
which offers vastly improved performance and easier setup of
the devices. Additionally the Netconf connector is optional,
making Netconf invisible for the suite if not installed.
Thus change the suite to query the netconf topology for a
list of configured devices.

This adds a configuration variable USE_NETCONF_CONNECTOR that
allows the netconf-ready suite to use a weaker condition for
the "Netconf is ready" check that does not depend on netconf
connector. The default of the variable is True, which makes
the netconf-ready suite to depend on the netconf connector
and it shall be set to False for test jobs which don't use
the netconf connector.

[0] https://wiki.opendaylight.org/view/OpenDaylight_Controller:Config:Examples:Netconf#Spawning_netconf_connectors_via_topology_configuration

Change-Id: Iac8bd7502014d0f897b3e92b351c2f8f61da9199
Signed-off-by: Jozef Behran <jbehran@cisco.com>