services: nginx: Add support the 'upstream' module.

* gnu/services/web.scm (<nginx-upstream-configuration>): New record type.
(<nginx-configuration>): Add new field upstream-blocks.
(nginx-upstream): New function.
(default-nginx-config): Add upstream-list parameter.
(nginx-service): Add optional upstream list keyword argument.
* doc/guix.texi (Web Services): Document the new nginx-upstream-configuration
data type and changes to the nginx function.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>