Remove redundant python-apt install

[jasdeep-hundal]

This implements the first half of issue #2284

Ansible automatically installs the python-apt package when using
the 'apt' Ansible module, if python-apt is not present. This patch
removes the (unneeded) explicit installation in the Kubespray
'preinstall' role.

Current Ansible code can be seen here: https://github.com/ansible/ansible/blob/7d721c1ec5d4066547c4b7bf51d452d956ae366a/lib/ansible/modules/packaging/os/apt.py#L888
And it's been there since 2013: ansible/ansible#4079

Fairly confident that this behavior won't change upstream, and even it does, the python-apt install in Kubespray won't work as is since it is using the 'apt' module that depends on the 'python-apt' package.

Tested by uninstalling python-apt/python-apt-common/python3-apt from an Ubuntu 16.04.3 Server, running Kubespray against it, and then validating a functioning deployment + presence of the python-apt package.

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[jasdeep-hundal]

CLA signed (that count as a reply? :))

[ant31]

ci check this