Ubuntu: Disable auto-mounting partitions

Permanently disable auto-mounting partitions to avoid problems with
writing to a USB drive.

BUG=b:529258692
TEST=manual

Change-Id: I4729909089f46e44f5c5bd489a71fd8c7c4fccc1
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/satlab/+/8703418
Commit-Queue: Tomasz Jakubowski <jakubowskit@google.com>
Reviewed-by: Rafał Gaweł <rafalgawel@google.com>
Tested-by: Tomasz Jakubowski <jakubowskit@google.com>