fix(pipeline): prefix hotflix slug with alpha chars to conform to k8s specs

Follow-up fix for !170 (merged)

Fixes deployment errors with hotfix branch names starting with numbers: Error: Service "2-89-x-backend-app" is invalid: metadata.name: Invalid value: "2-89-x-backend-app": a DNS-1035 label must consist of lower case alphanumeric characters or '-', start with an alphabetic character, and end with an alphanumeric character (e.g. 'my-name', or 'abc-123', regex used for validation is '[a-z]([-a-z0-9]*[a-z0-9])?')