tests: lsblk --json output changes mountpoint key to mountpoinst []

[blackboxsw]

Proposed Commit Message

Ubuntu Jammy output from lsblk --json now contains
'mountpoints': [...] instead of 'mountpoint' for children devs.

Let our integration test handle either case.

Test Steps

 CLOUD_INIT_PLATFORM=lxd_vm CLOUD_INIT_KEEP_INSTANCE=true CLOUD_INIT_CLOUD_INIT_SOURCE=IN_PLACE CLOUD_INIT_OS_IMAGE=bionic::ubuntu::bionic  .tox/integration-tests/bin/pytest tests/integration_tests/modules/test_disk_setup.py::TestDeviceAliases

-- or Jammy --
 CLOUD_INIT_PLATFORM=lxd_vm CLOUD_INIT_KEEP_INSTANCE=true CLOUD_INIT_CLOUD_INIT_SOURCE=IN_PLACE CLOUD_INIT_OS_IMAGE=jammy::ubuntu::jammy .tox/integration-tests/bin/pytest tests/integration_tests/modules/test_disk_setup.py::TestDeviceAliases

Checklist:

• My code follows the process laid out in the documentation
• I have updated or added any unit tests accordingly
• I have updated or added any documentation accordingly