Add flag to disable EPEL

Add DIB_EPEL_DISABLED flag that allows installation of the EPEL repo,
but to have it disabled by default.  This will help when you have
unavoidable EPEL dependencies, but want to make sure you only pull
specific things in with "--enablerepo" calls when installing those
packages.

Change-Id: Iedf6167a7cd69418255ebbee095aea04c50d73fd

diskimage_builder/elements/epel/README.rst

@@ -1,9 +1,13 @@
 ====
 epel
 ====
+
 This element installs the Extra Packages for Enterprise Linux (EPEL)
 repository GPG key as well as configuration for yum.
 
+Note this element only works with platforms that have EPEL support
+such as CentOS and RHEL
+
 DIB_EPEL_MIRROR:
    :Required: No
    :Default: None
@@ -11,3 +15,9 @@ DIB_EPEL_MIRROR:
                  before running bin/disk-image-create. This URL should point to
                  the directory containing the ``5/6/7`` directories.
    :Example: ``DIB\_EPEL\_MIRROR=http://dl.fedoraproject.org/pub/epel``
+
+DIB_EPEL_DISABLED:
+   :Required: No
+   :Default: 0
+   :Description: To disable the EPEL repo (but leave it available if
+                 used with an explicit ``--enablerepo``) set this to 1

diskimage_builder/elements/epel/pre-install.d/05-rpm-epel-release

@@ -36,6 +36,10 @@ else
     rpm -q epel-release || yum install -y $URL/$PKG_NAME
 fi
 
+if [ ${DIB_EPEL_DISABLED:-0} -ne 0 ]; then
+    yum-config-manager --disable epel
+fi
+
 DIB_EPEL_MIRROR=${DIB_EPEL_MIRROR:-}
 [ -n "$DIB_EPEL_MIRROR" ] || exit 0