Previously, all node vendor passthru methods required an exclusive lock
on the node to validate and start the task. This allows node vendor
passthru methods to specify require_exclusive_lock=False in their
passthru decorators to let the conductor acquire a shared lock on a
node.
Change-Id: I43cf43bc5c17f44a735e16c3c5cb744cf6911d27
Closes-Bug: #1481665