Maint/go mod and valid seperator

[Hermsi1337]

Changes:

• Switched separator from "," to "." since "," is not a valid label value
  * Valid label values must be 63 characters or less and must be empty or begin and end with an alphanumeric character ([a-z0-9A-Z]) with dashes (-), underscores (_), dots (.), and alphanumerics between
  • https://kubernetes.io/docs/concepts/overview/working-with-objects/labels/#syntax-and-character-set
• Moved to go modules
• Upgraded to go 1.12
• Removed deprecated function calls
• Add goreleaser for pushing images and building binary
• Push to master will create and push the latest tag
• stable- and versioned-tags will be deployed upon new tags in github

[martin-helmich]

Github cannot properly show the Files tab on my machine (probably too many files), so no inline comments:

• Regarding the . separator: This is an annotation not a label. The usual restrictions for label values should not apply to annotation values. AFAIK, using a comma in here should work just as intended.
• Regarding the Docker images: Do we really need a separate Docker tag containing the Go version with which the generator was built?

[Hermsi1337]

Regarding the seperator:
zomfg, you're ight.. o.O
I could swear that kubernetes yelled errors at me for the values containing a ,... maybe solstice.

Regarding the tag:
I thought a tag containing the go-version is great for someone who builds his own image on top of the base kubernetes-secret-generator-image.