1) Fix the invalid doc links according to the lastest OpenStack Website.
2) Replace them with the right ones.
Change-Id: I8d768802e25944a9761263cb2c80f5aaca317e96
CONTRIBUTING.rst includes bug report link, so it's useful for developer.
This commit also uses openstack instead of stackforge in README.
Change-Id: I6f2fb5a3cfc9154b48ae77dc1c8412df53679473