selenium: Restore sauce user/key configuration for wdio

selenium: Restore sauce user/key configuration for wdio

I removed these in e78484109ae7 because they seemed redundant
given that the library used by wdio-sauce-service already reads
these when present. However, our current version of this package
clobbers this unconditionally with config.user/config.key, thus
not letting it reach that default.

Also add sauceConnect:true which means developers can run
localhost tests against sauce labs if they so choose.

Change-Id: I8d31a23374b2b9b6cd6f1405c7a2f1001da08f1a