Make Boost an optional feature of Trilinos.

I have been unable to build Boost with some toolset/platform combinations. The
users I support don't need the Boost features in Trilinos, so I have been
manually hacking trilinos/package.py to disable boost. I would like to propose
that these changes be added to the released version.

+ Add boost as a variant, default to true so there is no change to existing
  behavior.
+ Move the CMake configure options related to Boost into a conditional block
  that is triggered by "if '+boost' in spec".