Refactor: scheduler: set red/yellow/green values when unpacking configuration

Previously, the globals for the value of "red", "yellow", and "green" were set
based on the "node-health-red", "node-health-yellow", and "node-health-green"
configuration options in unpack_config(), and then overridden in
pcmk__apply_node_health() if the node health strategy required different values.

Now, do it all at once in unpack_config() using a new function
unpack_node_health_values(). This also ensures that the correct values will be
available to char2score() immediately after unpacking.