Previously, _check_node() wouldn't catch the raise ValueError when
a drive was unmounted. Therefore the error would bubble up, uncaught,
and stop the shard cycle. The practical effect is that an unmounted
drive on a node would prevent sharding for happening.
This patch updates _check_node() to properly use the check_drive()
method. Furthermore, the _check_node() return value has been modified
to be more similar to what check_drive() actually returns. This
should help prevent similar errors from being introduced in the future.
Closes-Bug: #1806500
Change-Id: I3da9b5b120a5980e77ef5c4dc8fa1697e462ce0d
c26d67efcf