GCE: Network and Firewall: complete --check implementation
GCE: Static IP: complete --check implementation
GCE: Instance: implement extensive --check for disks, fix a bug on detach
GCE: Instance: extensive --check implementation
GCE: Instance: implement disks_ok in _check()
GCE: Instance: fail on project/region change if deployed
GCE: deployment.gce.region must always be specified
GCE: Instance: check that exactly one boot device is specified, fix booting from externally-managed disk
GCE: Firewall: add a sanity check for the rules specification
GCE: Instance: during deploy --check of a deployed machine, ensure that public IP address is up-to-date, fix disk attachment checking logic
GCE: Instance: add crude dependencies
GCE: Instance: decrease the number of GCE API calls mostly during deploy --check, by optimizing away tag and metadata refreshes