literal needs quotes or it will be parsed as a variable - which will not...

... exist

to avoid this:
'''
ocal:
    Data failed to compile:
----------
    No matching sls found for 'gitlab' in env 'base'
----------
    Rendering SLS "base:gitlab.packages" failed: Jinja variable 'postgresql' is undefined; line 93

---
[...]
      - openssh-server
      - python
      - python-docutils
      - redis-server
      - zlib1g-dev
      {% if salt['pillar.get']('gitlab:db_engine', postgresql) == 'postgresql' %}    <======================
      - libpq-dev
      {% endif %}
{% endif %}

{% if salt['pillar.get']('gitlab:use_rvm', False) %}
[...]