Refuse to schedule against missing suite branches

Signed-off-by: Zack Cerza <zack@redhat.com>
Merge pull request #373 from andrewschoen/orchestra-test

added a test for teuthology.orchestra.cluster.write_file
moved all the cluster.write_file tests into their own class; rewrote the existing test to have one assertion per test case

Signed-off-by: Andrew Schoen <aschoen@redhat.com>