allow locked and notify_audit as default fields

[astropuffin]

What happened:
AssertionError

bash-4.3# dogpush -c config.yaml diff
Diffing local files against datadog...
Traceback (most recent call last):
  File "/usr/local/bin/dogpush", line 3, in <module>
    from dogpush import dogpush
  File "/usr/local/lib/python2.7/site-packages/dogpush/dogpush.py", line 416, in <module>
    CONFIG = _load_config(args.config)
  File "/usr/local/lib/python2.7/site-packages/dogpush/dogpush.py", line 51, in _load_config
    set(DATADOG_DEFAULT_OPTIONS.keys()) == set())
AssertionError

What you expected to happen:
I expect to be able to set the following config in my config file (as per the example):

default_rule_options:
  notify_audit: True
  locked: True

And I expect that default option to propagate to my rules

How to reproduce it (as minimally and precisely as possible):
Set the following in the top level config file (along with all your other settings).

default_rule_options:
  notify_audit: True
  locked: True

run dogpush -c config.yaml diff

This PR moves the defaults from the non-overwritable list to the overwritable list. I have tested that this works locally with the diff arg, but not sure what other considerations this impacts.