deploy anbox-cloud-dashboard fail on AMI when AMI is already with pro subscription

[Chuan Li]

A customer tried to deploy anbox-cloud-dashboard 1.21/stable rev 307 on the machine with AMI ami-0c460fdf6a8a1edef, but it failed. The charm hooks fails at

subprocess.CalledProcessError: Command '['apt-get', '--assume-yes', '--option=Dpkg::Options::=--force-confold', 'install', 'anbox-cloud-dashboard-121']' returned non-zero exit status 100.

Although the machine/image already includes pro subscription, but just doesn't enable anbox-cloud by default.
This results in the client needing to manually enable anbox via 'sudo pro enable --access-only anbox-cloud', in order to have the repo and repo's credential files created.

If charm detects a machine with a pro subscription, it is expected to be able to perform a 'pro enable anbox-cloud' operation automatically.