When the vnf maintenance plugin's implementation of
project_instance_pre() is called, vnf instance id is likely to be empty
because, for instance, the stack creation itself has not been done for
some reason.
This resolves a FT failue: vnfm.test_vnf_placement_policy.VnfTestCreate
.test_vnf_with_placement_policy_invalid. _get_instances_with_openstack()
is called on the cleanup path (delete_vnfd), though the vnf instance id
is not present because create_vnf must have failed due to the invalid
placement policy. And you must have seen the following error message in
the tacker-server log: "heatclient.exc.HTTPNotFound: ERROR:
The Stack (None) could not be found.", which masked the expected
tacker.extensions.vnfm.HeatClientException: "["invalid"] is not an
allowed value ["anti-affinity", "affinity", "soft-anti-affinity",
"soft-affinity"].
Related-Bug: #1886213
Change-Id: I8bd2f7e021476470df6131a7ac8abd402e6e1bd1
3798c9f925