Remove requirement for 'default' repository storage path

With the introduction of Gitaly Cluster this requirement's inflexibility has become more pronounced. As it stands today every GitLab environment must always have a repository storage path named 'default' else it will fail when trying to configure the rails nodes.

When configuring Cluster as documented users will then have 2 repository storage paths but only 1 will ever be getting used (storage-1). This requirement also makes automation around building environments and project imports more complicated. We should look at removing the need to always have a default storage path.