close the requests.Session() object used by dcos_api_session

[margaret]

High-level description

What features does this change enable? What bugs does this change fix?

The requests.Session() object is not used as a context manager so it needs to have close() called explicitly.

Corresponding DC/OS tickets (obligatory)

These JIRA ticket(s) must be updated (ideally closed) in the moment this PR lands:

• https://jira.mesosphere.com/browse/DCOS_OSS-3479

Related tickets (optional)

Other tickets related to this change:

• QUALITY- Foo the Bar so it stops Bazzing.

Related dcos-launch and dcos PRs

Is this change going to be propagated up into another repo? Test the change by bumping the dcos-test-utils SHA to point to these changes to test it. Link the corresponding PRs here:

dcos/dcos#2860

Checklist for all PRs

• Included a test which will fail if code is reverted but test is not. If there is no test please explain here:
• Include a test in dcos-integration-tests in https://github.com/dcos/dcos or explain why this is not applicable:
• Include a test in https://github.com/dcos/dcos-launch or explain why this is not applicable:

Integration tests were run and

• AWS Cloudformation Simple passed (link to job: )
• Onprem-AWS passed (link to job: )
• Onprem-GCE passed (link to job: )

PLEASE FILL IN THE TEMPLATE ABOVE / DO NOT REMOVE ANY SECTIONS ABOVE THIS LINE

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[margaret]

enterprise integration test is https://jira.mesosphere.com/browse/QUALITY-2127

[jongiddy]

Not sure if this change will go ahead, given dcos/dcos#2860 (comment)

However, if it does, you should move the api.wait_for_dcos() into the try block. This ensures that the session is also closed if an exception occurs during the long-running wait_for_dcos(). The session is created in the create() method, so this is safe.