Explicily enable the stores used by devstack

Devstack currently relies on the default value of the `known_stores`
configuration option. This patch enables explicitly the default stores
used by devstack.

The real fix for the issue below will land in Glance. However, since the
default stores will be FS and HTTP we need devstack to enable Swift's as
well, which is required in the gates, hence this patch.

Partial-Bug: #1255556
Change-Id: Id9aab356b36b2150312324a0349d120bbbbd4e63
(cherry picked from commit 355fc866833e0bd83796da1c45e4f94b58d5f500)
Merge "Explicily enable the stores used by devstack" into stable/havana