[Buildbot] #3483: rewrite usage of virtualenv in firstrun docs

[Buildbot trac]

#3483: rewrite usage of virtualenv in firstrun docs
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Reporter:  rutsky     |      Owner:
    Type:  undecided  |     Status:  new
Priority:  major      |  Milestone:  undecided
 Version:  0.9.0b7    |   Keywords:
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 Currently we create virtualenv and do all command from __inside__ of
 virtualenv, which sounds horrible to me: virtualenv should be used as
 opaque directory with minimal usage of internal structure.

 Also these commands doesn't work on windows, since `pip` and other scripts
 are inside `Scripts/` directory, not `bin/`.

 I suggest to rewrite docs to use virtualenv activation as it was done in
 eight branch.

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Ticket URL: <http://trac.buildbot.net/ticket/3483>
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