Add setting to disable using default image for the people page.

#4814 

Added a checkbox in the os_profiels app seetings. In ``/cp/build/features/os_profiles``:
![selection_128](https://f.cloud.github.com/assets/4497748/1949335/0324947e-80fb-11e3-9ba7-d8a66ff3c5b8.png)

When disabling the default image we get (checkbox is checked):
![selection_130](https://f.cloud.github.com/assets/4497748/1949337/149f485c-80fb-11e3-8138-0562891a517d.png)

With the default image:
![selection_129](https://f.cloud.github.com/assets/4497748/1949338/2a48ba1c-80fb-11e3-8e70-0ba38f208824.png)