docs: Replace 'nova boot' with 'openstack server create'

Replace references to novaclient with OSC in the boot from volume guide.

This is essentially a revert of commit aa39641, which was a revert of
an earlier attempt at doing this that fell down because it didn't
reflect the changes in CLI parameters between the different tools.

Change-Id: Ic99440dd618243517f64506e3da88885fc2c44c9
Signed-off-by: Stephen Finucane <[email protected]>

Author: stephenfin