Could retry generating random MAC addresses for guest NICs on conflict

[bnaecker]

We currently generate a random MAC address for guest NICs, whether during a saga or in an explicit call to create a new NIC for an instance. These MAC addresses are guaranteed to be unique, currently within the scope of a VPC, by an index on the network_interface table. If there is a conflict, the entire request fails with a 500-level error. That's a bad experience for the customer, and the MAC address space is not actually that large. (Currently just over 1M addresses, but soon down to just under 1M.)

This issue tracks adding some kind of retry strategy to this, similar to what we do for selecting a random IPv6 subnet for a VPC Subnet. It should noted that this issue may be obviated by choosing an overall different strategy for allocating these addresses. For example, generating a random address, and then doing a linear scan to find the next free address, might be an overall better strategy that could be done entirely in the database.

[bnaecker]

Closed by #1135