Use Netopeer2 for NETCONF key authentication test

Use Netopeer2 for NETCONF key authentication test

Netopeer image (sdnhub/netopeer) used for NETCONF key authentication
test is 6 years old and its SSH library (libssh v0.6.4) does not
support the 'rsa-sha2-512' signing algorithm used by controller.

Use Netopeer2 (sysrepo/sysrepo-netopeer2) instead, which is also used
for NETCONF callhome tests, to support more modern crypto algorithms
and fix a test failure.

Change-Id: Ib205dfdeaa8c7001d430c955d8629c096818d75a
Signed-off-by: Sangwook Ha <sangwook.ha@verizon.com>