Change docs to use non-public images by default

In Juno, the ability to make public images will be admin-only by
default. This patch changes areas of the documentation that
aren't explicitly known to be working with admin-level credentials
to use is-public=False in their image uploads. This encourages good
security practice, and will hopefully avoid bug reports from users
who don't have access trying to make public images.

Change-Id: I8ca05c7507e1e63d39c927388f95fc584dfbe34d
Partial-Bug: 1328302
Merge "Change docs to use non-public images by default"