Enable kubernetes startup probes to get status of pmm-agent using 'GET HTTP' verb

To allow the usage of httpGet with k8s probes: https://kubernetes.io/docs/tasks/configure-pod-container/configure-liveness-readiness-startup-probes/

To test:

• check that both curl -v http://127.0.0.1:7777/local/Status and curl -v -X POST http://127.0.0.1:7777/local/Status work;
• check that pmm-admin status, pmm-admin list, etc still work.