Concurrent backup operations using NFS driver may fail with
error 'busy or already mounted' when trying to mount the
share. Adding a config option 'backup_mount_attempts' to
specify the number of times to retry the mount operation
before raising the exception.
Closes-bug: 1813851
Change-Id: I35dcc5d61237fa65488eb9b2c8cbd26ee7b2adc1