Ignore undefined values in customizable filter

When using the customizable filter to modify variables, if a template
overrides file is shared between multiple sets of images, it is sometimes
possible that a customised variable does not exist for a set of images.
A specific example:

My template overrides file has this:

{% set ironic_inspector_packages_append = ["python-pip"] %}

Normally I build binary/centos images.

kolla-build -t binary

Sometimes however I build a bifrost centos/source image, because bifrost
does not support binary images.

kolla-build -t source bifrost

The ironic inspector image does not define ironic_inspector_packages for
the source build, since it does not need any.

{% elif install_type == 'source' %}
    {% if base_distro in ['debian', 'ubuntu'] %}
        {% set ironic_inspector_packages = ['iptables'] %}
    {% endif %}

{{ macros.install_packages(ironic_inspector_packages |
        customizable("packages")) }}

The install_packages macro copes with the undefined variable, however
the customizable filter does not handle the base variable being
undefined if a customised example exists.

This change allows the customizable filter to ignore undefined
variables, even if they have been customised.

Change-Id: Ibe2fc91f5b6ceee8f937dd73c235128b2db525b1
Closes-Bug: #1809491

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