Add storage_driver_password_file option for security

[basvandijk]

For security reasons it would be better not to pass secrets via the command line because other processes can read the command line. So I'm requesting a feature that besides the option:

-storage_driver_password=some-secret

we get the option:

-storage_driver_password_file=/run/keys/my-secret

[dashpole]

cc @timstclair security-related