Our images use 'scsi-target-utils' from EPEL and then can be built
without it. So let's disable this repository right after installing
package from it.
Images can enable EPEL with 'enable_extra_repos' macro.
Also added 'disable_extra_repos' macro as we have some images in a need.
Change-Id: Id70474e7602b531b63fdc11ae690fd9ed33def74