backups: add option to disable encryption
backups: add option to disable encryption

While the encryption is enabled by default in reasonable cases, allow the
user to disable it if he/she want to.

woju/qubes-core-admin,woju/qubes-core-admin,marmarek/qubes-core-admin,woju/qubes-core-admin,QubesOS/qubes-core-admin,marmarek/qubes-core-admin,QubesOS/qubes-core-admin,marmarek/qubes-core-admin,woju/qubes-core-admin,QubesOS/qubes-core-admin