k8s conformance: Sanitize major/minor version str

Since we use `kubectl version` to determine K8s client/server version
strings, the major/minor semver components might include some custom
suffixes, e.g. '1.13+abcdef' would be split into '1' and '13+abcdef'.
Avoid including the custom semver suffixes into the kube-conformance
image tag name by sanitizating the major/minor version strings.

JIRA: VAL-111

Change-Id: Iac4f719e4e982b90b50a23e5bcaeff4bda9fe104
Signed-off-by: Alexandru Avadanii <Alexandru.Avadanii@enea.com>