Currently, there is no way to supply custom pod filters to
exclude pods meeting arbitrary criteria when using drain
as a library.
This commit adds the ability for developers to add custom
filters to the drain operation when utilizing drain
as a library.
This commit exports certain types that were previously
private to allow for better code reuse.
This commit also adds appropriate unit tests.
Kubernetes-commit: 85004f030dc3dceb9d15f41fdd607545758d5da2
a28f944c90