Release all node locks during disk replace

This patch extends commit 7ea7bcf6 by releasing all node locks in disk
replace for the early release mode. The rationale behind this is:

- LUCreateInstance already releases all node locks while waiting for
  disk synchronization, and does an instance startup later
- WaitForSync only runs (for disk template 'drbd') 'lvs' and read
  /proc/drbd on the primary node, which should be (modulo bugs in LVM)
  safe for parallel run

In any case, the worst I could foresee is a node having N lvs commands
run in parallel on it, while being a primary for disk storage. Based on
create instance doing this safely, and the fact that burnin with more
than two instances per node is safe, I think this can be applied.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>