configurator: required and default argument 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>
core: add read-only layer type

This patch introduces Access field for buxton's configuration. This field
isn't strictly required, instead of another field, so get_ini_*
functions were extended for this purpose.

Signed-off-by: Alexey Perevalov <a.perevalov@samsung.com>
db/gdbm: reopen db for read only

This patch adds ability to read from write protected db.
Without it any get_* routines to such type of db will be failed at
stage of open gdbm file.

Signed-off-by: Alexey Perevalov <a.perevalov@samsung.com>