Improvements to exporter image, drone CI

[HarrisonWAffel]

Relates to: rancher/windows#223

• Removes slack integration steps from drone.yaml, from what I can tell this integration isn't used anymore
• Updates CI scripts
  • use ltsc2019 tags to be inline with how windows images are tagged throughout rancher
    • It should be noted that both 1809 and ltsc2019 tags have the exact same contents
  • remove hyper-v isolation build condition, as all official builds will come from CI, which ensures that the host id and release ID windows versions will always match
• Update the docker file
  • Removes invalid empty separators resolving several docker build warnings
  • Removed the use of entry.ps1, as the exporter dependency on wins is being removed. The entry point is now just invoking the downloaded binary directly with the same arguments passed by the rancher-monitoring chart
  • Updated the nanoserver stage as we no longer need to run powershell scripts, from my testing I can see that this reduces final image size by ~100MB
• When building the image in environments the version cannot be detected, automatically set the version to dev

Changing the Dockerfile to no longer invoke wins would be considered a breaking change when using this image in rancher monitoring (as the chart needs to also be changed), so we should cut a new release of this in the v0.1.x line

[aiyengar2]

when this is merged, it should be cut as a v0.1.0 since this version of our image will definitely not work with rancher/wins.