By default, this will put a commented out variable in your group_vars:

# custom__packages:
#   - "extra-apt-package"

Should your project need extra apt packages installed, just uncomment and put them in a list:

custom__packages:
  - "php7.1-cgi"
  - "php7.1-odbc"
  - "php7.1-snmp"
  - "php7.1-soap"

On the next provision, these will be installed during cleanup (so any standard prereqs, like php 7.1 itself, would already be installed beforehand)

/cc @jasoncomes