Change default to not use system-ghc unless otherwise specified.

[bitemyapp]

Justification here: #1125 (comment)

[bitemyapp]

Inverting this: https://github.com/commercialhaskell/stack/blob/master/doc/yaml_configuration.md#system-ghc

[bitemyapp]

Rejected per @snoyberg (user feedback overwhelmingly wants system GHC)

[snoyberg]

As a general note that applies here too: if you think the general consensus wouldn't be this direction, feel free to raise the point on the mailing list to garner more feedback. I'm just basing this on whom I've heard from.

[bitemyapp]

@snoyberg sent!