enabling client certs

[sebasmannem]

Trying to have stolon work with client certificates.
Please see #869 for more info.

[sebasmannem]

Fixes #869

[sebasmannem]

@sgotti I tested this and it works. Team is running larger tests, just to see if all works as expected.
The implementation adds a lot of options, like other auth methods (peer, ident and cert auth), setting hostssl and the option to have different local auth and different remote auth options.
This implementation is very directly configurable, and with that highly flexible.
We could add documentation how to deploy a client cert authorised setup.
Alternatively we could wrap all in one or 2 options which would be easier to setup and document, but would not be as flexible.
Let me know your thoughts please...

[sebasmannem]

@sgotti I intend to finalise this and my other commit over the next few weeks.
Let me know if you have any comments, ok?