Add --users option to update-admin-ssh-keys

This option will make the script create individual user
accounts instead of copying the keys into the shared
/home directory. User accounts are granted the
ability to sudo.

The old option to maintain passwords in a single account
has been preserved so as not to break any existing
deployments.

Change-Id: I250e6813ae20c9eccb0ce848d8964e5880c5d002
Merge "Add --users option to update-admin-ssh-keys"