Add osc-lib to requirements.txt
Add osc-lib to requirements.txt

Without this requirement the installation of
python-magnumclient in an environment where nothing requires
osc-lib will at the very least break the command line
OpenStack client.

Change-Id: I905e2864a185e3135f45b23d9c3b87143e890e15
Closes-Bug: #1661035

openstack/python-magnumclient