configurator: Add required option for get_ini_* functions

Buxton's configuration right now is dealing only with required fields:
Backend, Type, Priority, Description. But for read-only layer it's
better to introduce none mandatory field "Access" with default value.

Signed-off-by: Alexey Perevalov <a.perevalov@samsung.com>
shared: Add read-only layer property

Add the optional access property to buxton's configuration. Use this
property to set if a layer can be updated (read-write) or not (read-only).