Revert to default mirrors for CentOS
Revert to default mirrors for CentOS

The method we used for setting the mirror doesn't work right
any more. Let yum work out the fastest, which will likely by
aarnet anyway

Change-Id: If5048ccefdf8cd529e56126589c2d9916b0fe8b4

NeCTAR-RC/nectar-images,NeCTAR-RC/nectar-images