Added easy site configuration to choose virtual package implementations

[citibeth]

Added a simple way to configure a site to use a particular choice for virtual packages, while maintainig backwards compatibility. It may be used by putting in spack/init.py, for example:

 choose_hints = {'mpi' : 'openmpi'}
 from spack.concretize import DefaultConcretizer
 concretizer = DefaultConcretizer(choose_hints=choose_hints)

This produces the following result:

$ spack spec netcdf

Concretized
------------------------------
  netcdf@4.4.0%gcc@4.9.3~fortran~hdf4+mpi=linux-x86_64
      ^curl@7.46.0%gcc@4.9.3=linux-x86_64
          ^zlib@1.2.8%gcc@4.9.3=linux-x86_64
      ^hdf5@1.8.16%gcc@4.9.3~cxx~debug+fortran+mpi+shared~szip~threadsafe+unsupported=linux-x86_64
          ^openmpi@1.10.2%gcc@4.9.3~psm~tm~verbs=linux-x86_64
              ^hwloc@1.11.2%gcc@4.9.3=linux-x86_64
                  ^libpciaccess@0.13.4%gcc@4.9.3=linux-x86_64
                      ^libtool@2.4.6%gcc@4.9.3=linux-x86_64

on branches efischer/160301-ChooseHints, origin/efischer/160301-ChooseHints
branches efischer/160301-ChooseHints, origin/efischer/160301-ChooseHints

[tgamblin]

This one is a good idea -- I am actually going to be merging #120 tomorrow, which includes a more detailed form of concretization preferences. Can you look at that one and see if it will do what you need? One nice thing about it is that the prefs go in a config file, not in __init__.py

[citibeth]

I agree, #120 looks like it's heading in the right direction.