Encrypted Backups

We'd like to enable the backups that are taken and stored as files in Swift, to be encrypted.
We'll add some new config flags, to mark whether or not encryption is enabled. At this point, the encryption key will also be stored as a config value.
When configured to encrypt, the whole process will be streamed to/from Swift, doing backup->zip->encrypt->Swift and Swift->decrypt->unzip->restore all during streaming, so we don't use up more disk space than needed.