python-novaclient 16.0.0 removed the deprecated
list_extensions module [1] so this changes the
extensions command to use openstacksdk to get the
compute API extensions.
The functional test ExtensionTests.test_extension_list_compute
ensures this works.
[1] https://review.opendev.org/686516/
Change-Id: I9894bc395c0474aaa6494ac4534862efe4ea7984
Story: #2006769
Task: #37284