Rename max_droplet_size to max_package_size

In preparation for surfacing the max app bits package size as a
configurable attribute in cf-release, rename
ccng.packages.max_droplet_size to ccng.packages.max_package_size to more
accurately reflect its purpose.

Also, since ccng.droplet.max_droplet_size is not used in the code,
remove its declaration from config.