move writing sources.list to CloudConfig. add 'apt_preserve_sources_list'
Move the writing of sources.list to CloudConfig. This way we have access to the cloud-config user data. Then, allow the user to specify the archive mirror using 'apt_mirror' key.
Now, if specified in cloud-config, that is used, otherwise, we get one from the DataSource.
One other change here is adding 'apt_preserve_sources_list'. If set to true, then overwriting of the sources.list with the selected mirror will not be done.