Bluetooth: GATT: introduce volatile subscription flag

Some application protocol required non-persistente subscription
across connection even in bonded case.
Flag BT_GATT_SUBSCRIBE_FLAG_VOLATILE specify if subscription
must be remove during disonnection.

Change-Id: I1bc2bbbb4bc86f58905e44a7eb267ca0871f2fdb
Signed-off-by: Jonathan Gelie <jonathanx.gelie@intel.com>