Bluetooth: Mesh: Add more flexible APIs for provisioning bearers

Add the ability to track the provisioning bearer through an extra
parameter to link_open/close. Also introduce new public functions to
enable/disable specific provisioning bearers. This also means that one
now needs to explicitly enable provisioning bearers after calling
bt_mesh_init().

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>