GCE Guest Suite for Ubuntu Core

Provides a subset of Google Cloud's guest-agent functionality
for Ubuntu Core's strict-confinement architecture

What's included:
* Metadata server polling
* Startup script execution (sandboxed)
* Hostname management
* ssh authorized_keys sync onto one fixed, pre-existing account

 (ubuntu) via the golang ssh-key-sync helper. This is not technically

 user creation and nor is it OSLogin; it writes one declared file for one

 account that already exists.

What's not included (i.e. not achievable on Core):
- OSLogin (The PAM && NSS modules need to live in the base snap)
- Dynamic creation of arbitrary users via metadata
- osconfig style patch management (Core does not have apt or .debs)
- guest-configs udev/sysctl/systemd unit files (could hypothetically be delivered via the gadget snap instead)