Enable the SCL repository for CentOS

Enable the Software Collections (SCL) repository for CentOS. This
repository includes useful software (e.g. the Go Toolset) which is not
present in the main repository.

For example, Octavia uses a Go based testing tool and its CentOS-based
jobs got broken now with the update to CentOS 7.6 which no longer
provides golang.

Change-Id: Ic68a6d6cd7da41510e624b6bea7976d9a960af98
1 file changed