Remove get_metadata() from os_type classes

Remove get_metadata() from os_type classes

The metadata are collected in the constructor and are saved in the
meta variable. This way they can be accessed and altered by the
sysprep methods. There are cases where some metadata are not valid
and need to be updated after a sysprep action is performed. For
example, remove_user_accounts sysprep action may remove a user that
is present in the USERS metadata.