Remove letsencrypt-cluster-issuer from chart

[batpad]

Thanks for the catch from @dakotabenjamin in #249

The letsencrypt cluster issuer is defined here: https://github.com/developmentseed/osm-seed/blob/develop/osm-seed/templates/letsencrypt-issuer.yaml and this works fine if there is only one instance of osm-seed on the cluster. If there are multiple instances, this fail to install because the cluster issuer already exists.

I think the best solution is probably to remove the cluster-issuer from the osm-seed helm chart and add docs on setting that up as a separate helm chart that you can setup once on the cluster.

I was really hoping for people to not have to fiddle with too many things and be able to install everything needed with a single install of this helm chart - maybe we can add it as an option, but osm-seed should definitely not by default create the cluster issuer if there are multiple instances on a cluster. The best solution for now I think is to take it out, and install the Cluster Issuer separately on the cluster.

cc @Rub21 @geohacker

[batpad]

Turns out it's fine to have multiple issuers on the cluster (thanks @yuvipanda ), so we will go with the approach in #249 .