Add credential provider option to kubeadm

[alexmasi]

This allows passing in a config file to define a credential provider for the kubelet. This enables automatic auth refresh for pulling images from private gcp regs on a GCE VM.

[coveralls]

Pull Request Test Coverage Report for Build 8840466257

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 27 of 47 (57.45%) changed or added relevant lines in 3 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.05%) to 65.113%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
deploy/deploy.go	1	4	25.0%
cluster/kubeadm/kubeadm.go	24	32	75.0%
controller/server/main.go	2	11	18.18%

Files with Coverage Reduction	New Missed Lines	%
controller/server/main.go	1	37.67%

Totals
Change from base Build 8807012865:	-0.05%
Covered Lines:	4625
Relevant Lines:	7103

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💛 - Coveralls