Make centos reset-bls-entries behave the same as rhel

This makes 03-reset-bls-entries consistent with rhel so that the glob
match is *.conf, and a check is added to ensure that a rename is
actually required.

Change-Id: I4adff43cf7d4f31d939e6ddf37ac8d162ccd0db7

diskimage_builder/elements/centos/post-install.d/03-reset-bls-entries

@@ -21,10 +21,12 @@ pushd /boot/loader/entries
 
 machine_id=$(</etc/machine-id)
 
-for entry in *; do
+for entry in *.conf; do
     new_entry=$(echo $entry | sed "s/^[a-f0-9]*/$machine_id/")
     echo "renaming $entry to $new_entry for new machine-id"
+    if [[ $entry != $new_entry ]]; then
         mv $entry $new_entry
+    fi
 done
 popd