Support pod-selector for drain command

[docbobo]

This PR is related to issue #737. While it does not add support for custom drain commands, it adds support for configuring the pod-selector to be used for draining, which should be sufficient to address all the Longhorn related issues.

[jackfrancis]

@docbobo Also make sure to sign your commit (see https://github.com/kubereboot/kured/pull/788/checks?check_run_id=14582462611 for more info)

[docbobo]

Okay, tried to address both issues. Please have another look at the description and let me know if you want additional changes.

[jackfrancis]

code lgtm, looks like it's still complaining about an unsigned commit

[docbobo]

Yes, I noticed. But there is only one commit in this PR and that's verified. Maybe the result of the required rebasing? Do I need to recreate the PR?

[docbobo]

Nevermind. The commit was signed but missing the Signed-off-by. Now it should be correct.

[jackfrancis]

lgtm

@docbobo could you submit a helm chart change to expose this new feature?

https://github.com/kubereboot/charts

We'll merge that once this new feature is part of a released version of kured.