Add option for global NodeBalancer tags

[glennpratt]

To enable better tracking of resources in a given linode account, linode-ccm could add an optional list of tags to all NodeBalancers.

Often services are created implicitly with third-party software, but an operator would still like to apply some common tags to them across a given cluster, e.g. clusterId:mycluster.

General:

• Have you removed all sensitive information, including but not limited to access keys and passwords?
• Have you checked to ensure there aren't other open or closed Pull Requests for the same bug/feature/question?

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Feature Requests:

• Have you explained your rationale for why this feature is needed?
• Have you offered a proposed implementation/solution?

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