Switch to upstream RPM repos + armband repo-comp.

Switch to upstream RPM repos + armband repo-comp.

Instead of using linux.enea.com for ALL RPM packages for CentOS7,
by overriding FUEL_MIRROR in the Makefile, switch to using
EXTRA_RPM_REPOS variable to point to armband RPM repo-component.

This way, the bulk of RPM packages will be fetched from upstream,
instead of our mirror.

- we no longer have to sync and merge from upstream the RPM repos,
  but only maintain the reduced armband repo-component, currently
  hosting 2 packages (qemu-user-static and cobbler-grub-aarch64);
- it will lower bandwidth usage to armband mirrors;
- first step for merging with upstream the RPM repos;
- easier to integrate with the way OPNFV replaces YUM repos on FM;

Change-Id: I76c1ef3d308c099d523099ea96fb57539ef71e04