Don't create empty secret_key_path file

Currently, the file resource will create a secret_key_path even if
secret_key_content is nil (due to default action of create).  This
change adds some protection so that we effectively only manage
file perms/ownership when secret_key_content is unset.

If secret_key_content is nil (default), the dashboard itself will
create secret_key_path and populate it.

Change-Id: Id7bff6a82eba1e80bf9883db3c45ba06edde2d91
Closes-bug: #1285125
Merge "Don't create empty secret_key_path file"