Reduce Server update complexity to below 20

One of the goals to polish Heat code is to make sure the complexity is
below 20.  This patch refactors the Server resource implementation
regarding its update logic so that it meets this requirement.

This is a pure code refacto patch, so no test cases are modified.

Change-Id: I18c6ada4386c0a091ae1d369db91c6a3d6429f34
Merge "Reduce Server update complexity to below 20"