hack: pin the release image in build flags

But still leave an override env var so that it can be overridden. Use the following env var to pin the image while building the binary:
```
$ # export the release-image variable
$ export OPENSHIFT_INSTALL_RELEASE_IMAGE=registry.redhat.io/openshift/origin-release:v4.0"
$ # build the openshift-install binary
$ ./hack/build.sh
$ # distribute the binary to customers and run with pinned image being picked up
$ ./bin/openshift-install create cluster...
```
The only way to override it would be to use an env var OPENSHIFT_INSTALL_RELEASE_IMAGE_OVERRIDE while running the openshift-install binary.

hack/build.sh

@@ -28,6 +28,10 @@ export CGO_ENABLED=0
 case "${MODE}" in
 release)
 	TAGS="${TAGS} release"
+	if test -n "${OPENSHIFT_INSTALL_RELEASE_IMAGE}"
+	then
+        	LDFLAGS="${LDFLAGS} -X github.com/openshift/installer/pkg/asset/ignition/bootstrap.defaultReleaseImage=${OPENSHIFT_INSTALL_RELEASE_IMAGE}"
+	fi
 	if test "${SKIP_GENERATION}" != y
 	then
 		go generate ./data

pkg/asset/ignition/bootstrap/bootstrap.go

@@ -30,10 +30,13 @@ import (
 
 const (
 	rootDir              = "/opt/tectonic"
-	defaultReleaseImage  = "registry.svc.ci.openshift.org/openshift/origin-release:v4.0"
 	bootstrapIgnFilename = "bootstrap.ign"
 )
 
+var (
+	defaultReleaseImage = "registry.svc.ci.openshift.org/openshift/origin-release:v4.0"
+)
+
 // bootstrapTemplateData is the data to use to replace values in bootstrap
 // template files.
 type bootstrapTemplateData struct {