Adds RemoveLabels() to remove all labels from a node that contains the
supplied search string. Uses this to remove node-feature-discovery
labels prior to re-applying newly discovered labels.
This behavior applies to consecutive runs of node-feature-discovery and
whatever restrictions are placed on it. For example if a whitelist is
specified that changes the labels that are discovered, any labels that
do not match the whitelist will be removed. Adds a small note to the
readme to point out this fact.
Adds tests to main_test.go to test the behavior of AddLabels and
RemoveLabels.
