Pipelines: Support DAG scheduling and dynamic child pipelines

This change also adds a code path through the spack ci pipelines
infrastructure which supports PR testing on the Spack repository.
Gitlab pipelines run as a result of a PR (either creation or pushing
to a PR branch) will only verify that the packages in the environment
build without error.  When the PR branch is merged to develop,
another pipeline will run which results in the generated binaries
getting pushed to the binary mirror.

var/spack/repos/builtin.mock/packages/gcc/package.py

@@ -14,6 +14,9 @@ class Gcc(Package):
 
     version('1.0', '0123456789abcdef0123456789abcdef')
     version('2.0', '2.0_a_hash')
+    version('3.0', '3.0_a_hash')
+
+    depends_on('conflict', when='@3.0')
 
     def install(self, spec, prefix):
         # Create the minimal compiler that will fool `spack compiler find`