tree: Increase readability of TreeEnvironment

The TreeEnvironment uses default-dict representation, which uses
__repr__ on items. This is not really human friendly and more
importantly it contains dynamic data (object id) which changes per
execution and we are using environment in fingerprint, which does not
really work well for TreeNodeDebug. This patch replaces the __repr__
with __str__ which improves the readability and avoids the dynamic data.
Signed-off-by: NLukáš Doktor <ldoktor@redhat.com>

avocado/core/tree.py

@@ -78,8 +78,17 @@ class TreeEnvironment(dict):
         return cpy
 
     def __str__(self):
-        return ",".join((super(TreeEnvironment, self).__str__(),
-                         str(self.origin), str(self.filter_only),
+        # Use __str__ instead of __repr__ to improve readability
+        if self:
+            _values = ["%s: %s" % _ for _ in self.iteritems()]
+            values = "{%s}" % ", ".join(_values)
+            _origin = ["%s: %s" % (key, node.path)
+                       for key, node in self.origin.iteritems()]
+            origin = "{%s}" % ", ".join(_origin)
+        else:
+            values = "{}"
+            origin = "{}"
+        return ",".join((values, origin, str(self.filter_only),
                          str(self.filter_out)))