Fixes yum proxy configuration bug

[piotrkpc]

This fixes yum proxy configuration bug. Before this perfkit command:

pkb.py --cloud=OpenStack --run_uri=browbeat --ignore_package_requirements=True --openstack_floating_ip_pool=public --machine_type=p.c2r2 --image=Centos-7 --http_proxy=http://IP.IP.IP.IP:8080 --openstack_image_username=centos --openstack_network=browbeat_private --https_proxy=http://IP.IP.IP.IP:8080 --benchmarks=fio --openstack_volume_size=1 --os_type=rhel --timing_measurements=runtimes

fails with yum error:

Options error: Error parsing "proxy = '\'"http://10.10.10.1:8080";\''": URL must be http, ftp, https, socks4, socks4a, socks5 or socks5h not ""

We don't need quotation marks neither semicolon at the eol in /etc/yum.conf

[tohaowu]

Would you resolve the code conflict?