Adding support for 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.

Blueprint: encrypted-backups
Change-Id: I05447306f4249bfd8e02de7b008ebda3387b2fbd
Merge "Adding support for encrypted backups."