synchzonize: allow use_ssh_args to take from ansible.cfg

I have some important things set in `ssh_args`, like `-F custom-ssh-config`, which I would like to apply during `rsync` as well as during `ssh`.  This patch adds an optional argument to `synchronize`, named `use_ssh_args`, which allows you to opt-in to this.

I've left the patch as off-by-default, to maintain compatibility, although I strongly believe that this shouldn't even be an option, and that it should just use `ssh_args` everywhere.

For anyone blocked by this issue: The patch is different for ansible 1.7.2 (stable release at time of writing), but the whole new module (as of this commit) Just Works when backported, so you can just use that.