[TESTS] cleanup tests and remove reference to ubuntu16

[TESTS] cleanup tests and remove reference to ubuntu16

the basic_vm test was historically called ubuntu16...
as we are now delaing more with ubuntu18 or ubuntu20, the reference
to the version16 is a bit old

- Replace ubuntu16 by basic_vm
- Use ubuntu20 instead of Ubuntu16
- Replace onap.small by m1.small to be natively compatible with any Openstack
  (default flavor)

Issue-ID: TEST-299

Signed-off-by: mrichomme <morgan.richomme@orange.com>
Change-Id: I69f573354c87c44c10581861450d48244db0c7df