nix_expr: Allow to specify 'inline' keyword arg.

There are various places in NixOps that are generating nix expressions
by string concatenation, using nixops.util.make_nix_string() and passing
it to the commandline of various Nix commands. This is no longer needed
when we're able to properly inline using the pretty printer.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
deployment: Format command arguments using py2nix.

This switches everything in _eval_flags(), build_configs(),
destroy_resources() and set_argstr() to py2nix() with the new inline
keyword argument.

We no longer need make_nix_string() in nixops.util anymore, thus by now
it's no longer existing anymore.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>