This patch removes the tobiko.openstack.neutron._cidr module that was
used to generate CIDRs whenever tobiko was going to create a new subnet.
That method had problems when several tobiko workers running in parallel
tried to create subnets. Collisions happened sometimes, resulting in the
creation of two subnets with a common CIDR that could fail if they were
connected to a common router.
This patch uses the neutron subnet pool feature [1]. When Tobiko creates
a subnet, it will not calculate a free IPv4/IPv6 CIDR, but will
associate that subnet to common Subnet Pools, shared by the different
tests. When the subnet is created, it is neutron responsibiltiy to
allocate a free CIDR for that subnet, based on the created Subnet Pools.
The method that creates the Subnet Pools uses an
@oslo_concurrency.lockutils.synchronized decorator to guarantee that two
Tobiko workers do not create the same Subnet Pool at the same time.
[1] https://docs.openstack.org/neutron/latest/admin/config-subnet-pools.html
Change-Id: I3b09e468310e06f8c4b7d7dbec02e4b2e6f67530
a6e7a522df