Change order for yum-repos file in centos7 element

Due to changes in centos7 element to use package-installs the order
of yum-repos in pre-install.d needs to be changed, to make yum repos
available before package-installs is called to install packages.

Change-Id: I47b584506465d90247f30bad515a32323e52180e
Merge "Change order for yum-repos file in centos7 element"