20.10 Release spice_agent_enabled doesn't have a sane default

[Adam Dyess]

https://api.jujucharms.com/charmstore/v5/nova-compute-322/archive/templates/queens/nova.conf

The nova.conf template doesn't have a sane default for the `spice_agent_enabled` if the relation data isn't present. Of course i should have updated nova-cloud-controller charm first so the relation data would be there, but by default this should set this value to "None"

agent_enabled = {{ spice_agent_enabled }}