openstack/common directory was used to keep files from oslo-incubator.
There is no file from oslo-incubator now, so don't need handle
particular case for openstack/common.
Note: method split_path will be merged in oslo.utils[1], then we
can remove this directory forever.
[1] https://review.openstack.org/#/c/313996/
Change-Id: I60628204f928753ad930c0a2da631473c7f4fdc3