support new git:sync command

[ltalirz]

dokku 0.23 comes with a git:sync command dokku/dokku#4297

This replaces the dokku-clone plugin; a fix for #88 is already built in.

My suggestion would be to deprecate the dokku_clone ansible module and replace the dokku_git_sync module with one that uses the dokku internals (and update the corresponding docs to no longer point to the commercial git_sync plugin).

@josegonzalez if you would like to do any of this, please go ahead.
I might have some time next weekend

[josegonzalez]

I won’t have time to contribute to non-core projects for a bit as I’m wrapping up my commercial admin tool. Feel free to take this on.