Add ability to setup extra SSH authorized keys in user data. Specify these keys as a user data list property 'authorized_keys'. Defaults to configuring these for ubuntu and galaxy user, though this can be configured with the user data option 'authorized_key_users'.

Place new functionality in cm/boot/conf.py module as well. Move handling of nginx configuration and 'conf_files' user data property into this module.

Implement tests for this new functionality as well as for functionality moved into new cm.boot.conf module ('conf_files' and nginx configuration).