[Mesos Slave] Check available metrics for backwards compatibility

[dixonscottr]

What does this PR do?

Before submitting a metric, adds a check to ensure the metric exists in the dict returned when querying the /metrics/snapshot endpoint.

Motivation

The check was failing for versions < 1.0.0, since GPU metrics are only available since version 1.0.0

Testing Guidelines

An overview on testing
is available in our contribution guidelines.

Versioning

• Bumped the check version in manifest.json
• Bumped the check version in datadog_checks/{integration}/__init__.py
• Updated CHANGELOG.md. Please use Unreleased as the date in the title
  for the new section.
• If PR impacts documentation, docs team has been notified or an issue has been opened on the documentation repo

Additional Notes

This replicates the logic found in the mesos_master check

[ofek]

Thanks!