Reinstall instance: disallow offline secondaries

Currently, reinstallation of a DRBD instance with the secondary node offline does:

node1# gnt-instance reinstall -f instance1
Waiting for job 139053 for instance1...
Thu Nov 18 01:36:09 2010  - WARNING: Could not prepare block device disk/0 on node node3 (is_primary=False, pass=1): Node is marked offline
Thu Nov 18 01:36:09 2010  - WARNING: Could not shutdown block device disk/0 on node node3: Node is marked offline
Job 139053 for instance1 has failed: Failure: command execution error:
Disk consistency error

Since this fails anyway, let's check the secondary nodes, thus
preventing any modifications to the instance (e.g. OS type change):

node1# gnt-instance reinstall -f instance1
Waiting for job 139058 for instance1...
Job 139058 for instance1 has failed: Failure: prerequisites not met for this operation:
error type: wrong_state, error details:
Instance secondary node offline, cannot reinstall: node3

The patch needs modifications to the _CheckNodeOnline function, in order
to display meaningful messages ("Can't use offline node" would be very
confusing for an instance reinstall, since we didn't select a node
manually).

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>