rbd: Set client timeout to 30 seconds.

By default the client_mount_timeout setting in librados is 300 seconds,
but that causes the connect to the Ceph cluster to block for 5 minutes
if the Ceph cluster is not available.

This patch is not ideal, but it mitigates the problem for now.

At a later point all this librados/librbd code should go back to libvirt
again, but the current versions of libvirt in the distributions are
to old for all the features we require.

For now this should prevent the CloudStack agent blocking for 5 minutes
when the Ceph cluster isn't available.

This is also tracked at the Ceph tracker: http://tracker.ceph.com/issues/6507