Add egg-info to python distribution.

Currently, grpcio cannot be used as a dependency for egg packages
(https://github.com/grpc/grpc/issues/6939)

There is likely a better solution, but this is intended as a patch
for the 0.15.0 release.

PYTHON-MANIFEST.in

@@ -1,6 +1,7 @@
 recursive-include src/python/grpcio/grpc *.c *.h *.py *.pyx *.pxd *.pxi *.python *.pem
 recursive-exclude src/python/grpcio/grpc/_cython *.so *.pyd
 graft src/python/grpcio/tests
+graft src/python/grpcio/grpcio.egg-info
 graft src/core
 graft src/boringssl
 graft include/grpc