Allow beaker rake task to accept proper preserve-hosts options

In the past it just allowed true/false, but the current beaker accepts a series
of options, including 'onfail' which is what I was after. This change passes
throught the value specified in the env var only now.

Signed-off-by: Ken Barber <ken@bob.sh>
Merge pull request #1082 from kbarber/maint/stable/allow-preserve-hosts-pass-through

Allow beaker rake task to accept proper preserve-hosts options