Add suppport for changing password of symmetric vaults.

Allows changing passwords of symmetric waults, using a new variable
`new_password` (or the file-base version, `new_password_file`). The
old password must be passed using the `password` or `password_file`
variables that also received new aliases `old_password` and
`old_password_file`, respectively.

Tests were modyfied to reflect the changes.