selenium: Only load 'sauce' service when needed

selenium: Only load 'sauce' service when needed

Also, remove the redundant unprefixed `user` and `key` options.
These would be cleaner to set through `sauceConnectOpts`
as `sauceConnectOpts { username: …, accesskey: … }`, but even
that isn't needed because wdio-sauce-service already reads these
same environment variables directly. No further config needed.

Also remove the duplication of default variables we don't have
any particular preference for yet so that enjoy improvements to
these when upgrading.

Change-Id: I0d4802f83509b0b49f775560b508d9929ae9979e