tests: bluetooth: tester: Move bt_mesh_init call to BTP_MESH_INIT cmd

In some RPR tests we need to pass alternative composition data when
testing CDP128. When bt_mesh_init is called from mesh service
registration callback, we can't pass any arguments. Therefore we need
to move bt_mesh_init call out from mesh service registration to
BTP_MESH_INIT command.

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