enhance: make version output consistent

[jbrockopp]

Currently when executing the plugin, you'll see output that looks like this:

$ make docker-run

### Executing vela-kubernetes:local image
time="2021-03-01T15:04:07Z" level=info msg="Vela Kubernetes Plugin" code="https://github.com/go-vela/vela-kubernetes" docs="https://go-vela.github.io/docs/plugins/registry/kubernetes" registry="https://hub.docker.com/r/target/vela-kubernetes"
time="2021-03-01T15:04:07Z" level=fatal msg="no config action provided"
{
  "canonical": "v0.2.0",
  "major": 0,
  "minor": 2,
  "patch": 0,
  "metadata": {
    "architecture": "amd64",
    "build_date": "2021-03-01T09:03:49Z",
    "compiler": "gc",
    "git_commit": "32fe344ad5e5fdef5b1cdd243de0647d1b01b20e",
    "go_version": "go1.16",
    "operating_system": "linux"
  }
}
make: *** [docker-run] Error 1

Or like this:

$ make docker-run

### Executing vela-kubernetes:local image
{
  "canonical": "v0.2.0",
  "major": 0,
  "minor": 2,
  "patch": 0,
  "metadata": {
    "architecture": "amd64",
    "build_date": "2021-03-01T09:03:49Z",
    "compiler": "gc",
    "git_commit": "32fe344ad5e5fdef5b1cdd243de0647d1b01b20e",
    "go_version": "go1.16",
    "operating_system": "linux"
  }
}
time="2021-03-01T15:04:07Z" level=info msg="Vela Kubernetes Plugin" code="https://github.com/go-vela/vela-kubernetes" docs="https://go-vela.github.io/docs/plugins/registry/kubernetes" registry="https://hub.docker.com/r/target/vela-kubernetes"
time="2021-03-01T15:04:07Z" level=fatal msg="no config action provided"
make: *** [docker-run] Error 1

The second form of output was the intended behavior when the code was initially written.

This change will ensure that we always output the version information at the very beginning before any other output.

Also, I added a fallback default of v0.0.0 for the semantic tag value we pass to the plugin.

This will help prevent a panic in the event that tag is not set as intended.

[codecov]

Codecov Report

Merging #69 (4e15553) into master (32fe344) will decrease coverage by 0.12%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master      #69      +/-   ##
==========================================
- Coverage   47.97%   47.84%   -0.13%     
==========================================
  Files           9        9              
  Lines         371      372       +1     
==========================================
  Hits          178      178              
- Misses        191      192       +1     
  Partials        2        2              

Impacted Files	Coverage Δ
cmd/vela-kubernetes/main.go	0.00% <0.00%> (ø)

[KellyMerrick]

lgtm