[FLINK-17223][AZP] Clean up disk space on Azure-hosted builders

[rmetzger]

What is the purpose of the change

Due to a recent change in the machines provided by Azure, the available disk space has reduced on these machines (actions/runner-images#709).
This has caused some of our build profiles to run out of disk space.

Brief change log

• Add a conditional step, executed on the host (not within the container) that cleans up the disk space on azure provided machines.

Verifying this change

This change has been tested here: https://dev.azure.com/georgeryan1322/Flink/_build/results?buildId=302&view=results

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Automated Checks

Last check on commit 7c034ff (Sun Apr 19 06:10:40 UTC 2020)

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• No documentation files were touched! Remember to keep the Flink docs up to date!

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CI report:

• 7c034ff Travis: SUCCESS Azure: PENDING

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[dianfu]

@rmetzger Thanks a lot for the PR. LGTM. Have verified that the Python tests could pass in the private azure pipeline after applying this PR: https://dev.azure.com/dianfu/58b46dfa-b96b-4b3f-a321-ce7644c4503b/_apis/build/builds/31/logs/167.