Package fails to install when user already exists.

[Steve Wormley]

We have our system pre-create users before installing packages so the UIDs match across all of our systems, however elasticsearch fails to install due to error:

Preparing to unpack .../elasticsearch_1.7.3+dfsg-3_all.deb ...
Warning: The home dir /var/lib/elasticsearch you specified can't be accessed: No such file or directory
adduser: The user `elasticsearch' already exists. Exiting.
dpkg: error processing archive /var/cache/apt/archives/elasticsearch_1.7.3+dfsg-3_all.deb (--unpack):

It appears that the problem is the preinst control file:
 if ! getent user 'elasticsearch' > /dev/null 2>&1 ; then

This should be "getent passwd"

It appears this is still present in the 1.7.5-1 version as well.

Versions:
Description: Ubuntu 16.04.3 LTS
Release: 16.04

elasticsearch:
  Installed: (none)
  Candidate: 1.7.3+dfsg-3
  Version table:
     1.7.3+dfsg-3 500
        500 http://archive.ubuntu.com/ubuntu xenial/universe amd64 Packages