Add automated releases to Sonatype Nexus through CI

[hectcastro]

After finalizing a decision on https://github.com/geotrellis/maml/issues/85, review the following change sets and add automatic release publishing to Sonatype Nexus:

• https://github.com/geotrellis/geotrellis-gdal/commit/2ed934578284007cb17491b94253625c76a01135
• https://github.com/raster-foundry/raster-foundry/commit/2677735cf4d9b791d601bf489d3177a77249efc6

In addition, include a release issue template like https://github.com/open-apparel-registry/open-apparel-registry/pull/451.