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.  Since there is currently no support in the cf-release templates for this config, the impact of the rename is minimal.

Also, since ccng.droplet.max_droplet_size is not used in the code, remove its declaration from config.