db: Fallback to read-only gdbm_open

This patch adds ability to read from write protected db. Without this
change, any get_* routines to a db without write access will fail at
gdbm_open.

Signed-off-by: Alexey Perevalov <a.perevalov@samsung.com>
shared: Add doc string for readonly layer property
test: Fixup set_label test