Implement TOSCA Block Storage

Create TOSCA BlockStorage node type and its mapping in Heat. Introduce
relationship template which will also provide a base for implementing
relationship templates section in the TOSCA template. Introduce tests for
generator/translator code.

implements blueprint tosca-blockstorage-type
Closes-Bug: #1351129
Closes-Bug: #1354632

Change-Id: I7df198ea45875031557e2607d932f14f272bf2e1