[mpagent] Wait for disk to be attached before mounting
[mpagent] Wait for disk to be attached before mounting

Bug: 723130
Change-Id: Id3a5c7d9097f1a65da849af095e270948087f3e6
Reviewed-on: https://chromium-review.googlesource.com/1223067
Commit-Queue: smut <95deb7aa15d4c615299e47e7688551456568b1fb@google.com>
Reviewed-by: Andrii Shyshkalov <a30c74fa30536fe7ea81ed6dec202e35e149e1fd@chromium.org>

luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go