Don't overwrite local config files

[gapple]

The copy task overwrite option defaults to false, and means that files will be overwritten if the timestamp of the source file is newer (https://www.phing.info/docs/guide/trunk/CopyTask.html). This results in local changes being destroyed if any changes are made upstream.

This pull request adds a conditional based on the existence of the project.local.yml file, so that the files are only copied the first time setup is run.

[grasmash]

Should we also be checking for the existence of local.settings.php and local.drushrc.php before copying those files as well?

[gapple]

Each could be split into separate conditions, but I was making the assumption that a user would run setup:drupal:settings (or another task that depends on it), rather than individually copying the example files manually.

I think it's okay to assume that if a user does a manual copy of the project.yml file, they know to copy the other example files when needed as well.

[gapple]

I found that properties.xml also contains a command to copy the default.local.settings.php file. I'm not completely sure why, but in case something is expecting that behaviour I wrapped it in the same conditional.

I then tried making the setup:drupal:settings task only copy files if all of them were missing:

<if>
  <not>
    <or>
     <available />
     <available />
     <available />
    </or>
  </not>
  <then>
</if>

but that will always fail if the local.settings.php file is created by properties.xml.

Which lead me to break out each file into a separate check + action, which is more verbose, but more understandable:

<if>
  <not>
    <available />
  </not>
  <then />
</if>
<if>
  <not>
    <available />
  </not>
  <then />
</if>