Workaround changed LVM behaviour

The vgreduce command has changed behaviour from when we initially
wrote the code (2.02.02 versus 2.02.66, 4 years delta):

- if there are LVs which will be impacted, it requires --force
- otherwise refuses to proceed, but it still returns exit code 0

We handle this by looking to see if it returns "Wrote out consistent
volume group" (behaviour unchanged), or if it complains about
"--force"; in the case it didn't complete, we retry the operation.

We improve a bit the checking of "vgs", as it uses to fail silently
and we didn't detect it.

New tests for this function should test, I believe, all the expected
variations; at the least we now have data files with the expected
output.

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