Move env vars to configuration settings

[delner]

We'd like service, env, tags, and version to be configurable from Datadog.configure as well as from their respective environment variables.

This pull request moves these into Datadog::Configuration::Settings to make them configurable.

[delner]

Something things I'd still like to do with this before its ready:

• Add deprecation warnings to c.tracer when passed tags, env to use new settings instead.
• Add documentation explaining how to set tags, env, version, service.
• Possibly move log and debug from c.tracer as well, while we're deprecating options.

[brettlangdon]

so far looks good to me

[delner]

Decided to hold off on deprecation warnings for now; want to give this a thorough treatment but the changes are probably too much for this PR.

Should be ready for a final review pending CI passing.

[delner]

Looks like there was a bug where setting Datadog::Logger.debug_logging = false would ignore the value and leave debug logging on. Fixed this as well.

EDIT: Never mind, this is evidently intentional behavior for custom loggers. The test for this was doing too many things at once and conflating things a bit; refactored it a bit for correctness and clarity.

[marcotc]

!