Add facility for plugins to register web routes

Rather than special-casing github and sql in zuul-web, add a
registration method to Connection to allow a Connection to register
routes.

Provide two types of WebHandler classes - a 'raw' class and a 'driver'
class. The driver class prepends '/connection/{connection}' to the
provided path.

Shift the github webhook and SQL web methods to use the plugin interface.

Change-Id: I065937b20447248e7894e1dfeec269faa6fd30d2
8 files changed