[FLINK-15646][K8s]Configurable K8s context support.

[zhengcanbin]

What is the purpose of the change

One would be forced to firstly use kubectl config use-context to switch to the desired context if working with multiple K8S clusters or having multiple K8S "users" for interacting with the specified cluster, so it is an important improvement to add an option(kubernetes.context) for configuring arbitrary contexts when deploying a Flink cluster. If that option is not specified, then the current context in the config file would be used.

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

• Dependencies (does it add or upgrade a dependency): (yes / no)
• The public API, i.e., is any changed class annotated with @Public(Evolving): (yes / no)
• The serializers: (yes / no / don't know)
• The runtime per-record code paths (performance sensitive): (yes / no / don't know)
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Yarn/Mesos, ZooKeeper: (yes / no / don't know)

Documentation

• Does this pull request introduce a new feature? (yes / no)
• If yes, how is the feature documented? (not applicable / docs / JavaDocs / not documented)

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Last check on commit b8552e9 (Wed Jan 29 07:31:11 UTC 2020)

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CI report:

• b8552e9 Travis: SUCCESS Azure: FAILURE
• 6d0b595 Travis: CANCELED Azure: FAILURE
• 9e9f56a Travis: SUCCESS Azure: SUCCESS

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[zhengcanbin]

cc @wangyang0918 @tisonkun

[wangyang0918]

@zhengcanbin The changes looks really good to me. I just have some minor comments.

1. The module in the commit title should be k8s or kubernetes. Maybe "[FLINK-15646][k8s] Make kubernetes context configurable" is better.
2. Even the abbreviation k8s is reasonable in the commit title to leave more characters for other useful information. I suggest to avoid in the code, documentation. This is not forced, and you could leave it as you want.

@tisonkun could you take another look and help to merge?

[zhengcanbin]

@zhengcanbin The changes looks really good to me. I just have some minor comments.

1. The module in the commit title should be `k8s` or `kubernetes`. Maybe "[FLINK-15646][k8s] Make kubernetes context configurable" is better.

2. Even the abbreviation `k8s` is reasonable in the commit title to leave more characters for other useful information. I suggest to avoid in the code, documentation. This is not forced, and you could leave it as you want.

k8s is more shorthand and used commonly in an informal context, sounds reasonable for the commit message.

[tisonkun]

Thanks for your contribution @zhengcanbin . I left some comments.

Beside, Azure fails on a known issue https://issues.apache.org/jira/browse/FLINK-15811. I'd suggest you rebase on nightly master and retest.

[zhengcanbin]

Thanks for your contribution @zhengcanbin . I left some comments.

Beside, Azure fails on a known issue https://issues.apache.org/jira/browse/FLINK-15811. I'd suggest you rebase on nightly master and retest.

Thanks, @tisonkun ! Another commit was pushed to fix the comments.