Vagrantfile - resiliently parse env variable

[tallowen]

env seems to be an array for me which means it cant have ".split" called on it. Here is an example:

SALT_NODE_ID=servo-master1
SALT_FROM_SCRATCH=true

Testplan:
vagrant up seems to work now

cc @aneeshusa do you mind reviewing this?

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This change is

[aneeshusa]

@tallowen this is actually my fault (and the second time I've done this). There's already an open PR for this at #394 so I'm going to close this but please feel free to work on one of the other open issues!