Make netconf node tcp-only leaf have default value

Make netconf node tcp-only leaf have default value

Netconf node tcp-only leaf defines if we should use SSH or TCP as an
underlying protocol for netconf session to a netconf device.

Currently it does not have any default value defined. That means if we
forgot to fill in tcp-only field when configuring netconf mount point,
netconf topology implementation will not know what protocol to use and
will throw NPE. However, it would be more user friendly to have some
default protocol defined. We should aim to use SSH by default as it is
more secure protocol than just plain TCP.

Make tcp-only field default to false.

JIRA: NETCONF-488
Change-Id: I99d17ad54db55089b9ec3e417d69e711058c760a
Signed-off-by: Jakub Morvay <jmorvay@frinx.io>