Require a subsystem for client connections

Require a subsystem for client connections

SSHClient does not really operate on the raw session, hence we should
not report it as set up after authentication.

Each client is connected to a particular SSH subsystem, and hence we
require the desired name (for example "netconf", but can be anything).

Once we complete authentication, we request the specified subsystem.
When the subsystem is open, we attach it to the Netty channel and only
then do we report the SSHClient to be up.

JIRA: NETCONF-1108
Change-Id: I9210b2924a9b8239161c8f4c649db3677e63b5b3
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>