mcc_drain_err metric should not be served for removed nodes

Description of problem:

Seen in 4.13.0-rc.2, mcc_drain_err is being served for nodes that have been deleted, causing un-actionable MCDDrainError.

Version-Release number of selected component (if applicable):

At least 4.13.0-rc.2. Further exposure unclear.

How reproducible:

At least four nodes on build01. Possibly all nodes that are removed while suffering from drain issues on 4.13.0-rc.2.

Steps to Reproduce:

Unclear.

Actual results:

The machine-config controller continues to serve mcc_drain_err for the removed nodes.

Expected results:

The machine-config controller never serves{{mcc_drain_err}} for non-existant nodes.