Extract gnu-sed detection into a function

hack/lib/swagger.sh

@@ -127,21 +127,14 @@ kube::swagger::gen_api_ref_docs() {
   find . -type f | cut -sd / -f 2- | LC_ALL=C sort > .generated_html
   popd > /dev/null
 
-  if LANG=C sed --help 2>&1 | grep -q GNU; then
-    SED="sed"
-  elif which gsed &>/dev/null; then
-    SED="gsed"
-  else
-    echo "Failed to find GNU sed as sed or gsed. If you are on Mac: brew install gnu-sed." >&2
-    exit 1
-  fi
+  kube::util::ensure-gnu-sed
 
   while read file; do
     if [[ -e "${output_dir}/${file}" && -e "${output_tmp}/${file}" ]]; then
       echo "comparing ${output_dir}/${file} with ${output_tmp}/${file}"
 
       # Remove the timestamp to reduce conflicts in PR(s)
-      $SED -i 's/^Last updated.*$//' "${output_tmp}/${file}"
+      ${SED} -i 's/^Last updated.*$//' "${output_tmp}/${file}"
 
       # By now, the contents should be normalized and stripped of any
       # auto-managed content.