Bluetooth: tester: Refactor adding attributes to the GATT database

This patch makes adding attributes more clean.
Attribute UUID and user data are now added along with the attribute
in gatt_db_add function. Thanks to that, some static variables have
been deleted, and the code is more readable.

Change-Id: I510bd7a1a9ae0210dd90520212a340ee1a3b2b53
Signed-off-by: Mariusz Skamra <mariusz.skamra@tieto.com>