Automatic merge from submit-queue (batch tested with PRs 58375, 58306, 58386, 57420, 58035). If you want to cherry-pick this change to another branch, please follow the instructions <a href="https://github.com/kubernetes/community/blob/master/contributors/devel/cherry-picks.md">here</a>.

add semver metadata regex

**What this PR does / why we need it**: When building a release tag with optional semver metadata, the minor and major versions are not parsed correctly (empty strings). This fix adds optional support to the regex for the metadata component.

example:

```
~/w/g/s/k/kubernetes ❯❯❯ git tag v1.9.0+stable.0                                 fixes/semver_metadata ✭
~/w/g/s/k/kubernetes ❯❯❯ hack/print-workspace-status.sh                          fixes/semver_metadata ✭
STABLE_BUILD_GIT_COMMIT 63e2eacd
STABLE_BUILD_SCM_STATUS clean
STABLE_BUILD_SCM_REVISION v1.9.0+stable.0
STABLE_BUILD_MAJOR_VERSION 1
STABLE_BUILD_MINOR_VERSION 9
STABLE_DOCKER_TAG v1.9.0_stable.0
gitCommit 63e2eacd
gitTreeState clean
gitVersion v1.9.0+stable.0
gitMajor 1
gitMinor 9
buildDate 2017-12-19T21:08:48Z
```

**Which issue(s) this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close the issue(s) when PR gets merged)*:

**Special notes for your reviewer**:

**Release note**:
```release-note
NONE
```