Fix periodic NETCONF Call Home connection dropping

Fix periodic NETCONF Call Home connection dropping

Callhome devices make reconnection every hour due to key re-exchange,
which is part of SSHD implementation.

When a specific session is authenticated and activated,
in case of key re-exchange, authentication (invoking of doAuth() method)
does not need to be made again.

While we are at it, also make sure CallHomeSessionContext's constructor
does not have side-effects -- while this cannot quite happen since we do
not reuse ClientSessions, it is a needless leak.

JIRA: NETCONF-681
Change-Id: I824c92d230c7570570d5eed21d489c435bdc8b22
Signed-off-by: Ivan Martiniak <ivan.martiniak@pantheon.tech>
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>