Implement resize of cinder volumes

This patch adds update capabilities to volume size. The new size must be
bigger than old one (as supported by Cinder). If the volume is attached,
it is first detached, then resized and attached again to the same mount.

Known limitations:
user has to care for hirself about (un)mounting the volume from within the
instance OS and resizing the file system after resizing of the volume.

Change-Id: I7c54b18eb865d0786ec5e3f50c92aeabdaa80a64
Implements: blueprint update-cinder-volume