Adds skip_site_provisioner_update option for sites

[tomjn]

With this site I can adjust the site template provisioner and it won't overwrite it on provision with the latest from github

  wordpress-one:
    skip_provisioning: false
    repo: https://github.com/Varying-Vagrant-Vagrants/custom-site-template.git
    skip_site_provisioner_update: true
    hosts:
      - one.wordpress.test

Closes #2730

Checks

• I've updated the changelog.
• I've tested this PR
• This PR is for the develop branch not the stable branch.
• This PR is complete and ready for review.

[update-docs]

Thanks for opening this pull request! Make sure CHANGELOG.md gets updated with this change, additionally any docs that need updated can be found at https://github.com/Varying-Vagrant-Vagrants/varyingvagrantvagrants.org

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