Fix ratelimiting

Current master does not respect ratelimiting, since parsing of the
api-paste.ini is faulty. api-paste.ini limited user limiting by
setting a line as follows:
user:<user-id>:(GET, *, ".*", 4, minute)

This was passed to the Limiter as kwargs with "user" as a key.
Thus multiple user limiting is not possible as well as extracting
the id of the user was bound to fail, since we checked on the key
with startswith("user:")

An example config in the api-paste.ini has to look as follows:
limits = (POST, "*", .*, 10, MINUTE)
limits.<user-id1>:(GET, "*", .*, 4, minute)
limits.<user-id2>:(GET, "*", .*, 2, minute)

The code has been modified to be able to handle such configs

Fixes bug: 1210869

Change-Id: Iaec669b282a567d42fbe21a6f64e7bc99161053b
Merge "Fix ratelimiting"