Re-enable Swift3 middleware support in DevStack

Because *swift* in no more allowed in ENABLED_SERVICES (see
I78222d7ee2ea8a281962129d5bcf06c118469510) swift3 middleware to
swift could not be installed anymore. This patch thighten a check
to allow swift3 (but not swift) in ENABLED_SERVICES.

Change-Id: I194f66ae7e5163e0e6178561b09f55024fdaa607
Merge "Re-enable Swift3 middleware support in DevStack"