cmd.py: tweak plugin information querying logic

We don't want to unconditionally disable plugin querying just because
--conf is not given. That would mean that the query_plugins_info field
in the default paths would always be ignored, even if set to True.

This patch tweaks the logic so that we remember in options.conf the
actual name of the config file that we read from, whether it be from
--conf or from one of the default paths.

This way, the `test` command will still work out of the box, as commit
f68831b envisioned. But query_plugins_info is now checked if the user
has a configuration file (whether it be a default one, or one specified
through --conf).

Change-Id: Id71ad207f0419f7cc52eca174dd859d4182d7250
Story: 2000387

doc/source/execution.rst

@@ -76,10 +76,10 @@ jenkins section
   By default this is set to the system configured socket timeout.
 
 **query_plugins_info**
-  Whether to query the Jenkins instance for plugin info when a configuration
-  file is provided. If a configuration file is not provided `jenkins-jobs` will
-  ignore this setting and skip querying for plugin information. True by
-  default.
+  Whether to query the Jenkins instance for plugin info. If no configuration
+  files are found (either in the default paths or given through the
+  command-line), `jenkins-jobs` will skip querying for plugin information. True
+  by default.
 
 
 hipchat section

jenkins_jobs/cmd.py

@@ -187,6 +187,7 @@ def setup_config_settings(options):
     # Load default config always
     config.readfp(StringIO(DEFAULT_CONF))
     if os.path.isfile(conf):
+        options.conf = conf  # remember file we read from
         logger.debug("Reading config from {0}".format(conf))
         conffp = io.open(conf, 'r', encoding='utf-8')
         config.readfp(conffp)