[Buildbot] #3279: get rid of getent in ansible playbooks

[Buildbot trac]

#3279: get rid of getent in ansible playbooks
-------------------+-------------------------------
Reporter:  sa2ajj  |      Owner:
    Type:  task    |     Status:  new
Priority:  major   |  Milestone:  sys - on-bb-infra
 Version:          |   Keywords:  ansible
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 We currently use `getent` module to get information about home directory
 of the remote user.

 skelly [https://github.com/buildbot/buildbot-
 infra/pull/97#discussion_r32183050 notes] that it's possible to use
 `~user` instead.

 Test the functionality and get rid of `getent`.

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Ticket URL: <http://trac.buildbot.net/ticket/3279>
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