Normalize the weights instead of using raw values

The weight system is being used by the scheduler and the cells code.
Currently this system is using the raw values instead of normalizing them.
This makes difficult to properly use multipliers for establishing the
relative importance between two wheighers (one big magnitude could
shade a smaller one). This change introduces weight normalization so
that:

- From an operator point of view we can prioritize the weighers that
  we are applying. The only way to do this is being sure that all the
  weighers will give a value in a known range, so that it is
  not needed to artificially use a huge multiplier to prioritize a
  weigher.

- From a weigher developer point of view, somebody willing to implement
  one has to care about 1) returning a list of values, 2) setting the
  minimum and maximum values where the weights can range, if they are
  needed and they are significant for the weighing. For a weigher
  developer there are two use cases:

    Case 1: Use of a percentage instead of absolute values (for example, %
    of free RAM). If we compare two nodes focusing on the percentage of free
    ram, the maximum value for the weigher is 100. If we have two nodes one
    with 2048 total/1024 free, and the second one 1024 total/512 free they
    will get both the same weight, since they have the same % of free RAM
    (that is, the 50%).

    Case 2: Use of absolute values. In this case, the maximum of the weigher
    will be the maximum of the values in the list (in the case above, 1024)
    or the maximum value that the magnitude could take (in the case above,
    2048). How this maximum is set, is a decision of the developer. He may
    let the operator choose the behaviour of the weigher though.

- From the point of view of the scheduler we ensure that it is using
  normalized values, and not leveraging the normalization mechanism to the
  weighers.

Changes introduced this commit:

1) it introduces weight normalization so that we can apply multipliers
   easily. All the weights for an object will be normalized between 0.0 and
   1.0 before being sumed up, so that the final weight for a host will be:

    weight = w1_multiplier * norm(w1) + w2_multiplier * norm(w2) + ...

2) weights.BaseWeigher has been changed into an ABC so that we enforce
   that all weighers have the expected methods.

3) weights.BaseWeigher.weigh_objects() does no longer sum up the
   computer weighs to the object, but it rather returns a list that will be
   then normalized and added to the existing weight by BaseWeightHandler

4) Adapt the existing weighers to the above changes. Namely
    - New 'offset_weight_multiplier' for the cell weigher
      nova.cells.weights.weight_offset.WeightOffsetWeigher
    - Changed the name of the existing multiplier methods.

5) unittests for all of the introduced changes.

Implements blueprint normalize-scheduler-weights

DocImpact: Now weights for an object are normalized before suming them
up. This means that each weigher will take a maximum value of 1. This
may have an impact for operators that are using more than one weigher
(currently there is only one weigher: RAMWeiger) and for operators using
cells (where we have several weighers). It is needed to review then the
multipliers used and adjust them properly in case they have been
modified.

Docimpact: There is a new configuration option 'offset_weight_multiplier'
in nova.cells.weights.weight_offset.WeightOffsetWeigher

Change-Id: I81bf90898d3cb81541f4390596823cc00106eb20
Merge "Normalize the weights instead of using raw values"