doc: fix openstack rbd installation command

This cannot work because client.volumes.key is not a command

   ssh {your-compute-host} client.volumes.key

replace with a tee to allow for copy/paste as well as using the

   cat client.volumes.key

in the following lines.

Signed-off-by: Loic Dachary <loic@dachary.org>
Merge pull request #700 from dachary/master

doc: fix openstack rbd installation command