Validate that MAC addresses in agent config are sane

We rely on the user providing accurate information about the MAC addresses in the agent-config, because at the point we read it we haven't seen the hosts yet. However, if the user gets this wrong then chaos may ensue.

Once inventory is available, we should validate that the user has not:

• Specified MAC addresses that belong to two different agents in the same host config; nor
• Specified MAC addresses that belong to the same agent in two different host configs

and fail the install if they have.