Rename LXC containers to LXD during upgrade

[babbageclunk]

They were converted to LXD but still named things like 3/lxc/4. This requires changing a number of places:

• renaming the machines in the exported model (in import)
• renaming the containers in LXD (migrate-lxc)
• renaming the machine agents in on-machine config and tools (upgrade-agents)
• rewriting the init scripts to keep the machine agents running (for start-agents, but done in upgrade-agents)