Rework SslHandlerFactory

Rework SslHandlerFactory

Tracking down all the possible implementations and bridging of
SslHandlerFactor is not nice.

Turn SslHandlerFactory into an abstract class, with an explicit
fixed specialization.

This also allows us to put the configuration parsing bits to where
users can find them -- and create dynamic SslContexts as appropriate.

Furthermore this ends up differentiating the deprecated
netconf.client.SslHandlerFactory -- which becomes SslContextFactory and
we end up ditching the manual SSLEngine creation.

JIRA: NETCONF-1237
Change-Id: I2cd4721336f9867921123521d7b18d91a8743835
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>