[fix] add package 'which' to CentOS-7 boilerplate

Newer CentOS-7 images from https://images.linuxcontainers.org

 do no longer
include the which command.

Issue:

    $ sudo -H ./utils/lxc.sh cmd searx-centos7 ./utils/filtron.sh install all
    INFO:  [searx-centos7] ./utils/filtron.sh install all
    ...
    Install Go in user's HOME
    -------------------------

    download and install go binary ..
    ...
    -bash: line 1: which: command not found
    -->|ERROR - Go Installation not found in PATH!?!
    -bash: line 2: which: command not found

Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>