resources: file: Implement file attributes

Add owner which must be username or uid of the file owner, group which is
the group name or gid of the file, and mode which is the octal unix file
permissions.

Add separate implementation for Go 1.6 and lower.

test/shell/file-mode.sh

@@ -0,0 +1,19 @@
+#!/bin/bash -e
+
+set -x
+
+# run till completion
+timeout --kill-after=20s 15s ./mgmt run --yaml file-mode.yaml --converged-timeout=5 --no-watch --tmp-prefix &
+pid=$!
+wait $pid	# get exit status
+e=$?
+
+ls -l /tmp/mgmt
+
+test -e /tmp/mgmt/f1
+test -e /tmp/mgmt/f2
+test -e /tmp/mgmt/f3
+test $(stat -c%a /tmp/mgmt/f2) = 741
+test $(stat -c%a /tmp/mgmt/f3) = 614
+
+exit $e