selenium: Run wdio directly without grunt

selenium: Run wdio directly without grunt

Doesn't seem to add anything, except complexity.

This has the benefit of matching more closely the way the README
recommends running individual tests.

Also add a check for 'chromedriver' before running it.
Normally the -e and pipefail would suffice, but because
it runs in the background, the error can be buried,
hard to find, or even not cause exit code to be set
properly. Thus, do a simple 'hash' check that will
print a useful command and exit cleanly.

Before:
> $ something &
> [1] 57922
> -bash: something: command not found
> [1]+  Exit 127                something
> $ echo $?
> 0

After:
> $ hash something
> -bash: hash: something: not found
> (exit: 1) $ echo $?
> 1

Change-Id: Id95f18927b5443defe679a77a82c5cbdd127c716