Actually enable new feature of container-sync

I modified devstack to run newer version of container-sync in
commit d03915f, and cherry-picked to stable/icehouse.
However, $SWIFT_SERVICE_PROTOCOL variable is introduced after
stable/icehouse (commit 18d4778), therefore my cherry-pick patch did
not enable container-sync feature.

This patch replaces this variable with string value "http", so now
icehouse swift deployed by devstack works appropriately.

Change-Id: I3b509312846d29a5d14607d5eb561eb9545b28a5
Closes-Bug: 1378646
Merge "Actually enable new feature of container-sync" into stable/icehouse
Merge branch 'stable/icehouse' of https://github.com/openstack-dev/devstack into stable/icehouse