[SRU] race condition between startup script and cloud-init

[IMPACT] Roughly 40% of users using google start-up scripts with apt commands will fail.

[CAUSE] Cloud-init and the Google Startup Scripts processes may run at the same time. If the cloud-init apt cloud-config module is running at the start of the Google Startup script then it may fail.

[FIX] Start the Google Startup scripts after cloud-init has finished.

[REGRESSION POTENTIAL] The risk for regressions is low.

[TESTING]
1. Create a new image from the test PPA
2. Launch an instance with a startup script of "apt-get -y update; apt-get -y install --download-only ubuntu-desktop && touch /tmp/it_worked"
3. Wait for instance to come up; confirm that /tmp/it_worked exists.

[ORIGINAL REPORT]
The image ships with archive.ubuntu.com in the bits on disk. Cloud-init changes that to gce.clouds.archive.ubuntu.com and also runs apt-get update. The user's startup script in this case calls apt-add-repository and then apt-get update. When these overlap, problems arise. When they don't, the process works. We've tested this by adding a sleep command (e.g. 'sleep 30') to the start of the user startup script in our reproduction runs.