Inline AsyncSshHandler.startSsh()

There is just no reason to have this method split out, inline it to its
sole user -- which is making interactions a bit clearer. We also see
that there is a blocking call, holding up the caller until the
connection succeeds.

JIRA: NETCONF-905
Change-Id: Iae781a743e670498c9329162bb05a7475f32d370
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>