isos: Check maximum expected size

This patch allows the configuration to express maximum expected size for
ISOs created in createiso and extra_isos phases. If the image is larger
than this limit, a warning is emitted in test phase. The compose itself
is not affected in any way.

JIRA: COMPOSE-2824
Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>

tests/helpers.py

@@ -169,7 +169,9 @@ class DummyCompose(object):
         self.log_debug = mock.Mock()
         self.log_warning = mock.Mock()
         self.get_image_name = mock.Mock(return_value='image-name')
-        self.image = mock.Mock(path='Client/i386/iso/image.iso', can_fail=False, size=123)
+        self.image = mock.Mock(
+            path='Client/i386/iso/image.iso', can_fail=False, size=123, _max_size=None,
+        )
         self.im = mock.Mock(images={'Client': {'amd64': [self.image]}})
         self.old_composes = []
         self.config_dir = '/home/releng/config'