Fix dynamic import of CONF.volume_api_class

Fix dynamic import of CONF.volume_api_class

Modifies the import to actually happen when API() is called instead of
when the module is loaded.

This is necessary because CONF.volume_api_class is always evaluated as
the default value since the config file hadn't been parsed yet when the
module is loaded.

Nova fixed this problem a while ago for the compute driver in
https://review.openstack.org/#/c/14353/.

Change-Id: I4c3d6b4300298ad0ac4e40a7a1a815cbf88bb151
Closes-Bug: #1543219