add CASSANDRA_ENABLE_USER_DEFINED_FUNCTIONS

[MartinNowak]

This variable enables user defined functions. It will set the
enable_user_defined_function option of cassandra.yaml.
User defined functions are supported since cassandra 2.2.

[tianon]

I'm sorry, but honestly if we're going to do anything more with environment variables it would likely be in the opposite direction (which we won't do due to the backwards compatibility implications) -- modifying a YAML file via shell script is very fragile, and setting complex configuration in this way is not great. For custom configuration, the only/best recommendation is to provide an appropriate cassandra.yaml file to your running instance (via a new image with FROM / COPY in a short Dockerfile, via Docker Configs, via bind mount, etc).

Closing in favor of #160 + docker-library/docs#1317.