Call sync to ensure critical data makes it to disk

To be enhanced later with narrower fsync logic in the accounts daemon
itself when it actually finds a change and writes a new authorized keys
file.
Merge pull request #43 from jkaplowitz/master

Call sync to ensure critical data makes it to disk
Sync contents to disk before starting the image bundle.
Merge remote-tracking branch 'upstream/master'
Sync contents to disk before starting the image bundle.
Merge pull request #44 from jeremyje/master

gcimagebundle: Sync before device mapping changes.
Add pauses back to gcimagebundle.
Merge pull request #47 from jeremyje/master

Add pauses back to gcimagebundle.
Merge pull request #45 from yueluog/master

Add disk space check.