Currently, the HPA considers unready pods the same as ready pods when
looking at their CPU and custom metric usage.  However, pods frequently
use extra CPU during initialization, so we want to consider them
separately.

This commit causes the HPA to consider unready pods as having 0 CPU
usage when scaling up, and ignores them when scaling down.  If, when
scaling up, factoring the unready pods as having 0 CPU would cause a
downscale instead, we simply choose not to scale.  Otherwise, we simply
scale up at the reduced amount caculated by factoring the pods in at
zero CPU usage.

The effect is that unready pods cause the autoscaler to be a bit more
conservative -- large increases in CPU usage can still cause scales,
even with unready pods in the mix, but will not cause the scale factors
to be as large, in anticipation of the new pods later becoming ready and
handling load.

Similarly, if there are pods for which no metrics have been retrieved,
these pods are treated as having 100% of the requested metric when
scaling down, and 0% when scaling up.  As above, this cannot change the
direction of the scale.

This commit also changes the HPA to ignore superfluous metrics -- as
long as metrics for all ready pods are present, the HPA we make scaling
decisions.  Currently, this only works for CPU.  For custom metrics, we
cannot identify which metrics go to which pods if we get superfluous
metrics, so we abort the scale.