Add read_only field attribute

Some fields of an object are created by the database automatically
like 'id' in service object, thus should not be changed in general.

Add a read_only field attribute to specify such situation, which
can only be set once and can't be changed any more.

Change-Id: I25ba81ed7ec0d16782a59851944a14792d4c84ce
Simplify filter_scheduler.populate_retry()

This is the exactly what dict.setdefault() is useful for.

Change-Id: I356af8f87b84fb23383c06080b8fd373054b5949
Remove duplicate method

Duplicate method(_do_test_create_instance_above_quota)
are found in compute/test_servers.py. Remove it.

Change-Id: I0753992cadc40adce40999bb5ae8ecd503bd53bc
Handle string types for InstanceActionEvent exc_tb serialization

Commit 59a6cf233b538d6666740de4796fce25ed8265aa added code to handle
serializing non-str exception traceback objects, but didn't account for
unicode.  This change uses six.string_types to handle str and unicode
objects for the exc_tb argument.

Adds a new unit test for the unicode case and firms up two existing
tests for how traceback.format_tb is mocked (or shouldn't be in the case
of exc_tb being a str).

Closes-Bug: #1327476

Change-Id: Icc10b62a3f65610c50e86c0b366c2b70b1a0932d
Merge "Handle string types for InstanceActionEvent exc_tb serialization"
Merge "Add read_only field attribute"
Merge "Simplify filter_scheduler.populate_retry()"
Merge "Remove duplicate method"