SWARM Mode has it's own built in Load balancer, so if we want to leverage sticky sessions,
or if we would just prefer to bypass it and go directly to the containers (aka tasks), via
--label traefik.backend.disable.swarm.loadbalancer=true
then we need to let Traefik know about the underlying tasks and register them as
services within it's backend.
* Add ability to set authenticated user in request header
Some web applications provide the ability to authorize users based on
the authenticated from Basic Auth. This patch provides a way to set a
key to which the authenticated user can be set in the Header.
For example, if I set `HeaderValue = "X-WebAuth-User"` and authenticate,
my application will be able to read my user name from that header and
provide me with the proper access.
This fixes#802
Prior to this fix the documentation for the swarm example included
syntax that would fail with the following error.
`Error : flag needs an argument: --docker.domain`
This fix specifies flags using the `=` between the flag name and value.
Tested on: Docker version 1.12.2-rc1, build 45bed2c, experimental
