Unset disable_functions in php config, fixes #1127

[rfay]

The Problem/Issue/Bug:

OP #1127 reports that drush php doesn't work as expected, due to aggressive (server-expected) disable_functions in our php config.

How this PR Solves The Problem:

Turns off the disable_functions in php config

Manual Testing Instructions:

On a d8 project,

• make sure you have done a composer require drush/drush (so you have a local drush 9)
• Use webimage: drud/ddev-webserver: 20180925_php_ini_disable_functions in your config.yaml
• ddev start
• ddev ssh
• drush version should show 9.4
• drush php
• You should be able to do interactive php commands like echo "hi there"; and php should execute and return without errors.

Automated Testing Overview:

Related Issue Link(s):

OP #1127

Release/Deployment notes:

[andrewfrench]

Running through the manual review steps, all looks good.

• composer require drush/drush
• Update config.yaml to webimage: drud/ddev-webserver:20180925_php_ini_disable_functions
• ddev start
• ddev ssh
• drush version => 9.4.0
• drush php
• echo "ahoy"; => ahoy