[FLINK-14907] [filesystem] Make Azure Storage Key configurable via Environment Variable

[knaufk]

What is the purpose of the change

Make Azure Storage Key configurable via Environment Variable

Brief change log

• update documentation on credential configuration for ABS
• add EnvironmentVariableKeyProvider for Azure Blob Storage

Verifying this change

(Please pick either of the following options)

This change is a trivial rework / code cleanup without any test coverage.

(or)

This change is already covered by existing tests, such as (please describe tests).

(or)

This change added tests and can be verified as follows:

(example:)

• build Flink
• set Flink configuration as outlined in documentation
• set AZURE_STORAGE_KEY environment variable
• use wasbs:https:// for checkpoints or as file input for a testing job
• (test_azure_fs.sh can be modified to test this)

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): no
• The public API, i.e., is any changed class annotated with @Public(Evolving): no
• The serializers: no
• The runtime per-record code paths (performance sensitive): no
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Yarn/Mesos, ZooKeeper: no
• The S3 file system connector: no

Documentation

• Does this pull request introduce a new feature? yes
• If yes, how is the feature documented? docs, JavaDocs

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Automated Checks

Last check on commit ab1dcfb (Mon Dec 02 06:34:53 UTC 2019)

Warnings:

• Documentation files were touched, but no .zh.md files: Update Chinese documentation or file Jira ticket.

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• ❓ 1. The [description] looks good.
• ❓ 2. There is [consensus] that the contribution should go into to Flink.
• ❓ 3. Needs [attention] from.
• ❓ 4. The change fits into the overall [architecture].
• ❓ 5. Overall code [quality] is good.

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[StephanEwen]

Looks good, +1 to merge.

Nit: I think the optional wrapping here is a bit overkill. For such a local variable, just checking for null seems even easier.

[knaufk]

@StephanEwen Thank you for the review. I have addressed your comment and fixed some typos in the documentation.

It would help, if we can still merge this before the release branch is cut.

[StephanEwen]

Merging this...