This patch changes the handling of NodeFeatureRules so that one feature
name (say "cpu.cpuid") can hold different types of features (flags,
attributes and/or instances). Requiring features to choose one single
type has not been a limitation of the API itself (and there has been no
validation on this) but an implementation decision.
The new evalutation logic of match expressions is such that "flags" and
"attributes" are basically evaluated as an union - they are both maps
but "flags" just don't have any value associated with the key. However,
"instances" are handled separately as that is basically an array of
maps and needs to be evaluated in a different way (loop over the array
of instances and evaluate expressions against the attributes of each).
Because of this difference care must be taken if mixing "instance"
features with "flag" and/or "attribute" features.
Note that the API types or their validation is not changed - just the
implementation of how the NodeFeatureRules are evaluated.
