Allow fat charms to deploy large files

"Fat charms", those with large payloads such as tarballs for code dependencies, need to be able to deploy via amulet with those assets.  When amulet constructs a local charm as a bazaar branch, any file over 20MB in size is skipped by bzr by default.  This branch sets on a per branch basis the configuration add.maximum_file_size to 0, to override the skipping behavior and allow the files to be added and deployed.

Also add 'curl' as a dependency for sysdeps.