Fix spacing in usage_from_stdin and info_from_stdin (issue #24186).

If "a" is a bash array, then the syntax to append the contents of $line as a
new element to the array is a+=("$line"), not messages+=$line

Using the former syntax just seems to append to the first element, creating a
long string and thus losing newline information.

Fixing this allows us to drop some empty lines from invocations of
usage_from_stdin.

cluster/lib/logging.sh

@@ -106,7 +106,7 @@ kube::log::usage() {
 kube::log::usage_from_stdin() {
   local messages=()
   while read -r line; do
-    messages+=$line
+    messages+=("$line")
   done
 
   kube::log::usage "${messages[@]}"
@@ -129,7 +129,7 @@ kube::log::progress() {
 kube::log::info_from_stdin() {
   local messages=()
   while read -r line; do
-    messages+=$line
+    messages+=("$line")
   done
 
   kube::log::info "${messages[@]}"