cpu plugin: Fix ValuesPercentage to behave as documented.

The documentation claims that ValuesPercentage is only considered when
!ByState && !ByCpu. Fix the behavior to match this documented behavior.
This makes cpu_commit_without_aggregation much easier.

Bug: #820
cpu plugin: Mark the "active" state as available.

This fixes the !ByState && ByCpu case, i.e. when the user requests one
metric ("active") per CPU.

Bug: #820