Ensure service and env are strings

[marcotc]

2.0 Upgrade Guide notes
The configuration settings c.env and c.service now have to be Strings.

Motivation:
There's no reason to be lenient for these two core configuration settings, they can only be meaningful text provided by the user.

Additional Notes:

How to test the change?

For Datadog employees:

• If this PR touches code that signs or publishes builds or packages, or handles
  credentials of any kind, I've requested a review from @DataDog/security-design-and-guidance.
• This PR doesn't touch any of that.

Unsure? Have a question? Request a review!

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (c7ef57c) 98.06% compared to head (23a9645) 98.06%.

Additional details and impacted files

@@            Coverage Diff             @@
##              2.0    #3326      +/-   ##
==========================================
- Coverage   98.06%   98.06%   -0.01%     
==========================================
  Files        1243     1243              
  Lines       71873    71865       -8     
  Branches     3388     3386       -2     
==========================================
- Hits        70481    70473       -8     
  Misses       1392     1392              

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[ivoanjo]

👍 LGTM