Create base class 'common secret'. Move common data and logic from
'secret' to 'common_secret' class. This allowed adding abstraction layer
for easier adding new 'secret' objects in future.
Convert 'secret' class to child from basic 'secret_common' with 'data'
and 'file' properties.

Signed-off-by: Alexey Krasikov <alex-krasikov@yandex-team.ru>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>