(WIP) Provide ability to set field to a unique string by default.

This is work in progress.
It provides an alternative approach to callable defaults as discussed at https://github.com/edx/XBlock/pull/247.
It might make more sense to check for this at the top of the function, and then just use `field.write_to` to set the value on the block, so that the data gets included in the call to `set_many`.