This reverts commit 7e1b8a7261.
openstacksdk >=0.19.0 fixes the filtering problems leading to all
ports being deleted. However openstacksdk <0.21.0 has problems with
dogpile.cache so use 0.21.0 as a minimum.
Change-Id: Id642d074cbb645ced5342dda4a1c89987c91a8fc