maint: don't require write access when reading files

Change 'r+' to 'r' for open() when just reading.
The '+' is uneeded and distracting.

Change-Id: I8c611ab6d63e165f86e588992011f73f69565249

nova/tests/fake_libvirt_utils.py

@@ -104,7 +104,7 @@ def file_open(path, mode=None):
 
 def load_file(path):
     if os.path.exists(path):
-        with open(path, 'r+') as fp:
+        with open(path, 'r') as fp:
             return fp.read()
     else:
         return ''

nova/tests/test_libvirt.py

@@ -1989,7 +1989,7 @@ class LibvirtConnTestCase(test.TestCase):
                 return FakeVirtDomain(fake_dom_xml)
 
             def _fake_flush(self, fake_pty):
-                with open(fake_pty, 'r+') as fp:
+                with open(fake_pty, 'r') as fp:
                     return fp.read()
 
             self.create_fake_libvirt_mock()