release workflow: Add spack command for generating the .gitlab-ci.yml

This spack command adds a new schema for a file which describes the
builder containers available, along with the compilers availabe on
each builder.  The release-jobs command then generates the .gitlab-ci.yml
file by first expanding the release spec set, concretizing each spec
(in an appropriate docker container if --this-machine-only argument is
not provided on command line), and then combining and staging all the
concrete specs as jobs to be run by gitlab.

etc/spack/defaults/release.yaml

@@ -0,0 +1,16 @@
+# -------------------------------------------------------------------------
+# This is the default spack release spec set.
+# -------------------------------------------------------------------------
+spec-set:
+    include: []
+    exclude: []
+    matrix:
+        - packages:
+            xsdk:
+                versions: [0.4.0]
+        - compilers:
+            gcc:
+                versions: [5.5.0]
+            clang:
+                versions: [6.0.0, '6.0.0-1ubuntu2']
+    cdash: ["https://spack.io/cdash/submit.php?project=spack"]