Fix mesos_slave service check

[AlexandreYang]

What does this PR do?

Fix Mesos service check + Add tests.

Fix for #4054

Long term solution

The service check logic should be outside of _get_json util method that is used multiple times.
Currently, the logic is quite cumbersome because service check logic is inside the _get_json util method.

Review checklist (to be filled by reviewers)

• PR title must be written as a CHANGELOG entry (see why)
• Files changes must correspond to the primary purpose of the PR as described in the title (small unrelated changes should have their own PR)
• PR must have changelog/ and integration/ labels attached
• Feature or bugfix must have tests
• Git history must be clean
• If PR adds a configuration option, it must be added to the configuration file.

[codecov]

Codecov Report

Merging #4448 into master will increase coverage by 6.45%.
The diff coverage is 96.36%.

Impacted Files	Coverage Δ
mesos_slave/tests/common.py	100% <100%> (ø)	⬆️
mesos_slave/tests/conftest.py	100% <100%> (ø)	⬆️
mesos_slave/tests/test_integration_e2e.py	92% <100%> (ø)	⬆️
mesos_slave/tests/test_unit.py	100% <100%> (ø)
mesos_slave/tests/test_integration.py	100% <100%> (ø)
...os_slave/datadog_checks/mesos_slave/mesos_slave.py	84.73% <89.47%> (+14.89%)	⬆️
...checks_dev/tests/plugin/test_environment_runner.py
riakcs/datadog_checks/riakcs/riakcs.py
oracle/tests/test_oracle.py
...r/datadog_checks/openstack_controller/__about__.py
... and 810 more