Add check for pre-2.0 style hostname_callable config value

[dimberman]

This lets us pull the change back in to 1.10.x, so that by the time 2.0 is around people will have had time and notice to update, without reading (the now quite long) UPDATING.md.

Depends on #8463

---

Make sure to mark the boxes below before creating PR: [x]

• Description above provides context of the change
• Unit tests coverage for changes (not needed for documentation changes)
• Target Github ISSUE in description if exists
• Commits follow "How to write a good git commit message"
• Relevant documentation is updated including usage instructions.
• I will engage committers as explained in Contribution Workflow Example.

---

In case of fundamental code change, Airflow Improvement Proposal (AIP) is needed.
In case of a new dependency, check compliance with the ASF 3rd Party License Policy.
In case of backwards incompatible changes please leave a note in UPDATING.md.
Read the Pull Request Guidelines for more information.

[dimberman]

@jhtimmins could you please explain a bit more what this PR does and what bug it addresses?

[jhtimmins]

@dimberman This addresses an incompatibility between versions 1.10 and 2. For the hostname_callable config value, the notation changed from using colons (socket:getfqdn) to periods (socket.getfqdn).

This is a problem if you already have an airflow.cfg file (or unittest.cfg) from < 2.0, as airflow will continue to use the out-of-date values and throw an error. This branch does the following:

1. Adds shim code to replace the colon w/ a period.
2. Modifies the way that deprecated config values are swapped out. Previously, they did it with simple string comparison. This uses a regex to search for and replace the values, because it's possible to use a custom hostname_callable value that is formatted in the deprecated manner. Using a regex gives more control about what constitutes a deprecated value.
3. Adds FutureWarning alert when the deprecated style is in use.

[ashb]

This lets us pull the change back in to 1.10.x, so that by the time 2.0 is around people will have had time and notice to update, without reading (the now quite long) UPDATING.md. Nice

[ashb]

(Don't merge this yet, Github Actions haven't run yet)

[jhtimmins]

Depends on #8463. @dimberman mind adding that to the description?