Prevent task symlink vhost for Debian to failed.

When vhost has not been created (using apache_create_vhosts: no), the
tasks should not be executed, otherwise it'll failed as the vhosts.conf
file has not been created and cannot been linked.
Merge pull request #16 from guillaumededrie/fix/error-vhost-link

Prevent task symlink vhost for Debian to failed.