Proposal: Fix inconsistent DNS driver API implementations

## Summary

While comparing the LuaDNS driver to other DNS drivers, I noticed that there are a number of subtly incompatible implementations sharing an otherwise common API which limit portability between providers.

For example, some drivers use empty strings to represent apex/naked records (e.g. example.com.), while others use `None`. Both categories of drivers employ explicit unit testing to confirm their preferred `Record.name` value.

Similarly, some drivers want `Record.name` as a rooted FQDN (e.g. `"host.example.com."`), others as an unrooted FQDN (e.g. `"host.example.com"`), and still others as a bare host name (e.g. `"host"`).

## Proposal

Assuming that the **libcloud** DNS class/driver APIs are intended to provide a consistent interface for users and drivers should provide logic within themselves to adapt the common objects and interfaces for their respective services, I propose the following:

1. Add common logic to the `Record` class to standardize apex/naked record names on the empty string (or `None`).
2. Add common logic to the `Record` class to enforce that `.name` represents the host name, without the domain suffix. This would likely involve adding new `.fqdn` and `.hostname` properties to expose rooted and unrooted FQDN strings, respectively.
- For the sake of compatibility with existing uses, `__init__()` would accept a rooted/unrooted FQDN, a bare host name, or `None`.
3. ~~Refactor `Record.update()` to leverage common code to enforce the items above, while being kind to existing users.~~
4. Expand DNS driver unit tests to validate data sent *to* the provider. Presently, it appears that testing primarily validates the formatting of (canned) responses received *from* a provider.

Start by reading the DNS Record class and its update() method, then compare the existing DNS driver unit tests mentioned in the proposal. Trace how drivers format Record.name for provider requests and responses. Done requires an agreed common naming API, compatibility behavior, and tests covering data sent to providers, but the proposal does not identify specific files or settle the design.

Written by the indexing model from the issue text.