Reduce cloud/CoAP keepalive time for wifi

[tstellanova]

submission notes

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Problem

Some users report compatibility problems with wifi routers with 30 second timeout intervals

Solution

Reduce the CoAP keepalive interval for wifi devices

Steps to Test

Test periodic publishing with an Argon device

References

ch76216

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Completeness

• User is totes amazing for contributing!
• Contributor has signed CLA (Info here)
• Problem and Solution clearly stated
• Run unit/integration/application tests on device
• Added documentation
• Added to CHANGELOG.md after merging (add links to docs and issues)

[avtolstoy]

Nit: :)