Fix failing solr tests since travis-solr.sh switched to multi core by default
Fix legacy wrappers to use relative path if requested

Absolute path in these scripts mens they are broken in distributions,
this fixes it as we use relative by default and it made sense to honor this.
Make ezpublish:legacy:assets_install & ezpublish:legacybundles:install_extensions smarter with symlinks

Mostly same change as in:
https://github.com/symfony/symfony/pull/11312

The Symfony change was not backported, so this works best in combination with Symfony 2.6,
but will also slightly benefit users on Symfony 2.3 (at least for these commands)