Add option to skip EPEL & other repo installs

Add an option to skip the EPEL & other repo installs for rhel7 based
platforms.

This option can serve two purposes; firstly as described in
I834f20e9ceae151788cec3649385da1274d7ba46 during platform bringup, a
publically available EPEL might not be available.  This will allow you
to pre-configure a hand-built repo, etc. so you can continue testing.

The other thing is that in a CI system you might be frequently
building images and pre-installing EPEL/RDO etc.  In that case this is
just extra work.

Change-Id: I9809449f4a43fa9b547c6e3ca92722c7f6e66d6a
diff --git a/stack.sh b/stack.sh
index 9b811b7..e31cd60 100755
--- a/stack.sh
+++ b/stack.sh
@@ -263,9 +263,7 @@
 # Some distros need to add repos beyond the defaults provided by the vendor
 # to pick up required packages.
 
-if is_fedora && [[ $DISTRO == "rhel7" ]]; then
-    # RHEL requires EPEL for many Open Stack dependencies
-
+function _install_epel_and_rdo {
     # NOTE: We always remove and install latest -- some environments
     # use snapshot images, and if EPEL version updates they break
     # unless we update them to latest version.
@@ -295,18 +293,27 @@
     sudo yum-config-manager --enable epel-bootstrap
     yum_install epel-release || \
         die $LINENO "Error installing EPEL repo, cannot continue"
-    # EPEL rpm has installed it's version
     sudo rm -f /etc/yum.repos.d/epel-bootstrap.repo
 
     # ... and also optional to be enabled
     sudo yum-config-manager --enable rhel-7-server-optional-rpms
 
+    # install the lastest RDO
     sudo yum install -y https://rdoproject.org/repos/rdo-release.rpm
 
     if is_oraclelinux; then
         sudo yum-config-manager --enable ol7_optional_latest ol7_addons ol7_MySQL56
     fi
+}
 
+# If you have all the repos installed above already setup (e.g. a CI
+# situation where they are on your image) you may choose to skip this
+# to speed things up
+SKIP_EPEL_INSTALL=$(trueorfalse False SKIP_EPEL_INSTALL)
+
+if is_fedora && [[ $DISTRO == "rhel7" ]] && \
+        [[ ${SKIP_EPEL_INSTALL} != True ]]; then
+    _install_epel_and_rdo
 fi