fix openstack provider to handle only Cinder volumes

[mvladev]

What type of PR is this?
/kind bug

What this PR does / why we need it:
When the cloud-controller-manager is running with PV label initializing controller and NFS volume is created, it causes nil pointer dereference error.

Which issue(s) this PR fixes:
openstack variant of #68996

Special notes for your reviewer:
This the OpenStack variant of #70432 .

Does this PR introduce a user-facing change?:

Fix cloud-controller-manager crash when using OpenStack provider and PersistentVolume initializing controller  

/sig cloud-provider
/sig openstack

[mvladev]

/assign @NickrenREN

[idealhack]

/ok-to-test

[NickrenREN]

LGTM, thanks @mvladev

/approve

BTW: could you add UTs for openstack too just like you did for aws and gce ?
also can you send you PR here https://github.com/kubernetes/cloud-provider-openstack too ?

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: mvladev, NickrenREN

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[adisky]

lgtm

[mvladev]

@NickrenREN

BTW: could you add UTs for openstack too just like you did for aws and gce ?

I've been looking at this, but it seems that there are no UT at all in this provider - all tests require real OpenStack installation to test. Couldn't find any mock / fake clients to do so.

also can you send you PR here https://github.com/kubernetes/cloud-provider-openstack too ?

Of course.

[NickrenREN]

sgtm, thanks @mvladev

/lgtm