Make CDash reporting easier to use (#9357)

* Better default CLI arguments for CDash reporting

--log-format=cdash is now implied if you specify the --cdash-upload-url
option to spack install.

We also now default to writing CTest XML files to cdash_report/ when using
the CDash reporter if no --log-file argument was specified.

* Improved documentation on how to use the CDash reporter

lib/spack/spack/report.py

@@ -254,6 +254,10 @@ class collect_info(object):
     """
     def __init__(self, format_name, install_command, cdash_upload_url):
         self.filename = None
+        if cdash_upload_url:
+            self.format_name = 'cdash'
+            self.filename = 'cdash_report'
+        else:
             self.format_name = format_name
         # Check that the format is valid.
         if self.format_name not in valid_formats:

lib/spack/spack/reporters/cdash.py

@@ -58,7 +58,17 @@
 
 
 class CDash(Reporter):
-    """Generate reports of spec installations for CDash."""
+    """Generate reports of spec installations for CDash.
+
+    To use this reporter, pass the ``--cdash-upload-url`` argument to
+    ``spack install``::
+
+        spack install --cdash-upload-url=\\
+            https://mydomain.com/cdash/submit.php?project=Spack <spec>
+
+    In this example, results will be uploaded to the *Spack* project on the
+    CDash instance hosted at https://mydomain.com/cdash.
+    """
 
     def __init__(self, install_command, cdash_upload_url):
         Reporter.__init__(self, install_command, cdash_upload_url)