Follow the rest of OpenStack and consistently switch to
python3 as default python interpreter and drop python2.
The "functional" tox target was failing with Python 2
as cinder/master has cut the Python 2.7 compatibility.
"functional" now points to the default python3. An explicit
"functional-py37" target has been added as well.
Fix the functional jobs as well:
- move the LVM job to centos-8 and adapt it when needed;
- move the Ceph job to ubuntu-bionic, as there are no
CentOS 8 Ceph packages (and the CentOS 7 ones do not
provide Python 3 bindings).
Closes-Bug: #1853372
Change-Id: Iea4f4f53df7400248848399494564910d3870f63
08108b5964