Bluetooth: Mesh: Generate new public key for each provisioning session

Mesh Profile Spec v1.0.1 Section 5.4.2.3: "If the public key was not
available using an OOB technology, then the public keys are exchanged
between the Provisioner and the unprovisioned device. For each
exchange, a new key pair shall be generated by the Provisioner and the
unprovisioned device."

This allows passing MESH/NODE/PROV/BV-12-C.

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