Bluetooth: Mesh: Flush pending settings in bt_mesh_reset/_cdb_clear

Upon bt_mesh_reset call, all mesh modules has to clear the privisioning
and configuration data. None of the modules store or erase in settings
subsystem immediately, but put this on the mesh settings work. After the
settings work is scheduled, all stored data will be removed and the
device will eventually be unprovisioned. Until then, the device is not
completely unprovisioned, thus calling bt_mesh_prov_enable,
bt_mesh_provision (and bt_mesh_cdb_create) should not be allowed.

Struct bt_mesh_prov has a reset callback stating that after this
callback is called, the device has been reset and can be re-provisioned
again. Also, bt_mesh_reset API description states that after calling
bt_mesh_reset API, the device needs to reenable the provisioning layer
to be provisioned again. But this is not correct since the settings has
to be cleared before the device can be reprovisionined.

This commit makes bt_mesh_reset flush pending settings so that the
device can be reprovisioned immediately and the API will behave as
written in the description.

The same applies to bt_mesh_cdb_clear.

Signed-off-by: Pavel Vasilyev <pavel.vasilyev@nordicsemi.no>