See merge request !10

caucase.util.load_crl API changed in an incompatible way in caucase 0.9.9 .
This change fixes the API breakage and adds support for mor than one CRL
in kedifa.app.http .

By inserting other CA certificate as first one in the list, the problem is
reproduced in the test.

Depending on the environment, various exceptions can be risen, so be tolerant
regarding the exception type.

/reviewed-on !5

It can fail too fast, without giving caucase any chance to start really.

It's better to reply 400 Bad Request on malformed requests and do not pollute
log with exceptions.

Also improve message in case of failure.

/reviewed-on nexedi/kedifa!4

As updater is used in environment, which requires it to have certificates
available as fast as possible, add a prepare step and allow to launch it with
--prepare-only switch.

Thanks to this it is possible to run it with configuration file to provide
fallback or master certificates for all slaves without connecting to the
network, thus resulting in fast preparation.

/reviewed-on nexedi/kedifa!3

Also cover loop method.

It will make further testing much easier

If at least once certificate has been downloaded from KeDiFa it shall never
use again the fall-back, as otherwise it would result with a problem, that
next unsuccessful download from KeDiFa would result replacement with
fall-back.

In order to do so state file is introduced keeping list of overridden
certificates. As now there is critical path regarding fetching certificates,
the lock is created to avoid concurrent updates.

Fix one condition.

In order for further development and features create mixin.

Features:

 * by default runs with 60s sleep
 * allows to have master, updateable, certificate, which is used in case if
   specific certificate is not available

Having PRIMARY KEY on certificate.id is to strict -- as real uniques is
required on id + reference in certificate table.

capturer does not work in some of tests environment.