gluon-core: util: avoid unintended second return value from gsub()

gsub() returns the number of matches as its second return value. This
was unintendedly passed through by the util functions trim() and
node_id(). It can be presumed that this had no effect in practice, but
it can lead to surprising output when passing values to print() for
debugging.

package/gluon-core/luasrc/usr/lib/lua/gluon/util.lua

@@ -23,7 +23,7 @@ local function do_filter_prefix(input, output, prefix)
 end
 
 function M.trim(str)
-	return str:gsub("^%s*(.-)%s*$", "%1")
+	return (str:gsub("^%s*(.-)%s*$", "%1"))
 end
 
 function M.contains(table, value)
@@ -96,7 +96,7 @@ function M.exec(command)
 end
 
 function M.node_id()
-	return string.gsub(sysconfig.primary_mac, ':', '')
+	return (string.gsub(sysconfig.primary_mac, ':', ''))
 end
 
 function M.default_hostname()