[v2] blockstorage: add manage-existing and unmanage api call#3399

kayrus merged 5 commits intov2gophercloud/gophercloud:v2from

bp-v2-ea86f48-469ca6a-6fb7d11gophercloud/gophercloud:bp-v2-ea86f48-469ca6a-6fb7d11Copy head branch name to clipboard