Drop the devstack-single-node-centos-7 nodeset

OpenDev is preparing to remove centos-7 nodes on March 15[*]. This
change drops one nodeset definition which is the last remaining
reference on DevStack's master branch. Also clean up a lingering
reference to swift-dsvm-functional-py3 which no longer exists.

[*] https://lists.openstack.org/archives/list/openstack-discuss@lists.openstack.org/message/A2YIY5L7MVYSQMTVZU3L3OM7GLVVZPLK/

Change-Id: Icd487e1012263a9b0bc13b529d31ff2025108adf
(cherry picked from commit bc59eb81a74a240efa52d5e7a61b1a395bf8eefc)