add CombinatorialSpecSet class for taking cross-products of Specs.

- add CombinatorialSpecSet in spack.util.spec_set module.
  - class is iterable and encaspulated YAML parsing and validation.

- Adjust YAML format to be more generic
  - YAML spec-set format now has a `matrix` section, which can contain
    multiple lists of specs, generated different ways. Including:
    - specs: a raw list of specs.
    - packages: a list of package names and versions
    - compilers: a list of compiler names and versions

  - All of the elements of `matrix` are dimensions for the build matrix;
    we take the cartesian product of these lists of specs to generate a
    build matrix.  This means we can add things like [^mpich, ^openmpi]
    to get builds with different MPI versions.  It also means we can
    multiply the build matrix out with lots of different parameters.

- Add a schema format for spec-sets

lib/spack/docs/example_files/spec_set.yaml

@@ -0,0 +1,21 @@
+spec-set:
+    include: [ ape, atompaw, transset]
+    exclude: [binutils,tk]
+    packages:
+        ape:
+            versions: [2.2.1]
+        atompaw:
+            versions: [3.1.0.3, 4.0.0.13]
+        binutils:
+            versions: [2.20.1, 2.25, 2.23.2, 2.24, 2.27, 2.26]
+        tk:
+            versions: [8.6.5, 8.6.3]
+        transset:
+            versions: [1.0.1]
+    compilers:
+        gcc:
+            versions: [4.9, 4.8, 4.7]
+        clang:
+            versions: [3.5, 3.6]
+
+    dashboard: ["https://spack.io/cdash/submit.php?project=spack"]