Expose osprofiler middleware as entrypoint

Filter factories can be exposed via entrypoint which means you can
import osprofiler as

[filter:osprofiler]
use = osprofiler#osprofiler

This gives osprofiler more control over its middleware entrypoint and
means services don't need to go looking for it.

Change-Id: I88b9c69011a1abd9810ece139215bf4503a66954

setup.cfg

@@ -39,3 +39,5 @@ oslo.config.opts =
     osprofiler = osprofiler.opts:list_opts
 console_scripts =
     osprofiler = osprofiler.cmd.shell:main
+paste.filter_factory =
+    osprofiler = osprofiler.web:WsgiMiddleware.factory