Allows writing to config objects but the data is not consistent internally

[Sean Perry]

Use case:

     self.conf(['--blaa-foo', 'bar'])

     self.conf.blaa.foo == 'bar' # true
     self.conf['blaa']['foo'] == 'bar' # true

     self.conf.blaa.foo = 'new_value'
     self.conf.blaa.foo == 'new_value # true
     self.conf['blaa']['foo'] == 'new_value' # false!

The `__getattr__` call returns a GroupAttr object for `blaa`. When I write to `foo` that creates a member variable on `blaa`. Now, when `__getitem__` is called it does not check for an attribute on `blaa` but instead goes back to the `__cache`.

Either:

a) Writes are not permitted and `__setattr__` should be used to block them
b) Writes are ok and `__getitem__` should check `hasattr` before calling `_get`.

We came across this in a Keystone Middleware test.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/321889

[Haifeng.Yan (yanheven)]

if want to override some opt, can use conf.set_override() instead.

[OpenStack Infra (hudson-openstack)]

Change abandoned by ChangBo Guo(gcb) (<email address hidden>) on branch: master
Review: https://review.openstack.org/321889
Reason: This is no update more than 6 months, feel free to restore it if you want to continue work on this.

[Doug Hellmann (doug-hellmann)]

We should make the configuration object write-only (at least for defined configuration values).