Fix ratelimiting

Fix ratelimiting

Current master does now respect ratelimiting, since parsing of the
api-paste.ini was faulty. api-paste.ini limited user limiting by
setting a line as follows:
user:<user-id>:(GET, *, ".*", 4, minute) which was passed to the
Limiter as kwargs with "user" as a key. Thus multiple user limiting
was 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)

Fixes bug: 1206976

Change-Id: I4adbe3dbe3a0bd607d6e675f230b0442b08ec791