'Prepare client context
Version parameter accepts single version string or tuple of strings.
Compatible version can be obtained later using:
cctxt = _get_cctxt(...)
version = cctxt.target.version'

def _get_cctxt(self, version=None, **kwargs):

    if (version is None):
        version = self.RPC_DEFAULT_VERSION
    if isinstance(version, tuple):
        version = self._compat_ver(*version)
    return self.client.prepare(version=version, **kwargs)
