Add vagrant-disksize to required Vagrant plugins

When !702 (merged) was added, vagrant-disksize was not added to the Vagrantfile plugins, so fresh installs will fail if it is not already installed locally (see output below).

This ensures vagrant-disksize is installed when provisioning the box.

~/gitlab-development-kit$ vagrant up --provider=virtualbox --provision
Installing the 'vagrant-share' plugin. This can take a few minutes...
Fetching: vagrant-share-1.1.9.gem (100%)
Installed the plugin 'vagrant-share (1.1.9)'!
Installing the 'facter' plugin. This can take a few minutes...
Fetching: facter-2.5.6.gem (100%)
Installed the plugin 'facter (2.5.6)'!
Bringing machine 'default' up with 'virtualbox' provider...
==> default: Box 'ubuntu/xenial64' could not be found. Attempting to find and install...
    default: Box Provider: virtualbox
    default: Box Version: >= 0
==> default: Loading metadata for box 'ubuntu/xenial64'
    default: URL: https://vagrantcloud.com/ubuntu/xenial64
==> default: Adding box 'ubuntu/xenial64' (v20190918.0.0) for provider: virtualbox
    default: Downloading: https://vagrantcloud.com/ubuntu/boxes/xenial64/versions/20190918.0.0/providers/virtualbox.box
    default: Download redirected to host: cloud-images.ubuntu.com
==> default: Successfully added box 'ubuntu/xenial64' (v20190918.0.0) for 'virtualbox'!
There are errors in the configuration of this machine. Please fix
the following errors and try again:

Vagrant:
* Unknown configuration section 'disksize'.