Sysinfo: make the collection of installed packages configurable

So that users can decide to enable this costly but, depending on
use case, very useful piece of information.
Signed-off-by: NCleber Rosa <crosa@redhat.com>

avocado/sysinfo.py

@@ -26,6 +26,7 @@ except ImportError:
 from avocado import utils
 from avocado.linux import software_manager
 from avocado.core import output
+from avocado.settings import settings
 
 log = logging.getLogger("avocado.test")
 
@@ -324,20 +325,29 @@ class SysInfo(object):
     * end_job
     """
 
-    def __init__(self, basedir=None, log_packages=False):
+    def __init__(self, basedir=None, log_packages=None):
         """
         Set sysinfo loggables.
 
         :param basedir: Base log dir where sysinfo files will be located.
         :param log_packages: Whether to log system packages (optional because
-                             logging packages is a costly operation).
+                             logging packages is a costly operation). If not
+                             given explicitly, tries to look in the config
+                             files, and if not found, defaults to False.
         """
         if basedir is None:
             basedir = utils.path.init_dir(os.getcwd(), 'sysinfo')
 
         self.basedir = basedir
-        self.log_packages = log_packages
+
         self._installed_pkgs = None
+        if log_packages is None:
+            self.log_packages = settings.get_value('sysinfo.collect',
+                                                   'installed_packages',
+                                                   key_type='bool',
+                                                   default=False)
+        else:
+            self.log_packages = log_packages
 
         self.start_job_loggables = set()
         self.end_job_loggables = set()

etc/avocado/avocado.conf

@@ -3,3 +3,6 @@ base_dir = /usr/share/avocado
 test_dir = /usr/share/avocado/tests
 data_dir = /usr/share/avocado/data
 logs_dir = ~/avocado/job-results
+
+[sysinfo.collect]
+installed_packages = False