Fix an issue when user forgot combine custom passwords

Fixes an issue when user forgot to combine
'kolla_ansible_custom_passwords',
'kolla_ansible_default_custom_passwords' and own dictionary with
custom passwords in configuration files. Now
'kolla_ansible_extra_custom_passwords' should provide only user
custom passwords to add or override in the passwords.yml.

Change-Id: I4813a1f6ab9cb566596e806bd0ada6dff342d119
Signed-off-by: Maksim Malchuk <maksim.malchuk@gmail.com>