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Make OpenStack LBaaS v2 Provider configurable

Add option 'lb-provider' to the Loadbalancer section of the OpenStack
cloudprovider configuration to allow using a different LBaaS v2
provider than the default.



**What this PR does / why we need it**:
This PR allows to use a different OpenStack LBaaS v2 provider than the default of the OpenStack cloud.

**Which issue this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close that issue when PR gets merged)*: fixes #

**Special notes for your reviewer**:

**Release note**:

```release-note
Added option lb-provider to OpenStack cloud provider config
```