CA-113396: Destoy password secrets used in SR creation after the creation is complete.

When creating an SR, if XenCenter creates new secrets, then they should be destroyed
after the SR creation is complete. This is safe since any secrets in the device-config
are duplicated and unique ones are used for each PBD creation.

Signed-off-by: Mihaela Stoica <mihaela.stoica@citrix.com>