Update vault docs to include example to set env var in task

[walterscarborough]

Signed-off-by: Sarah McAlear smcalear@pivotal.io

[walterscarborough]

We thought that it was very hard to set up vault with a task, and that adding this example would help clarify that.

Thanks,

Walter and Sarah

[vito]

Hi! Thanks for taking the time to submit this, and sorry I never got around to giving feedback on this or merging it.

Recently I've rewritten the Vault documentation to be more...well, useful. I just realized one thing still missing is showing an example of how to set and use credentials, so I opened #174 and then remembered this PR. Since I can't merge it anymore and you've probably since moved on with your life, I figure I'll just close this and start on the #174 soon. Hope that's OK.

Thanks again!