Improve zuul scripts and introduce daily run of e2e conformance test

Created by: matofeder

This PR does the following:

• Replaces ospurge in cleanup play by forceclean as it is considered stable after several clean scripts improvements: #526, #498, #551, #550, #494
• Introduces abstract job k8s-cluster-api-provider-e2e-abstract that is used as a base for all defined e2e jobs
• Introduces k8s-cluster-api-provider-e2e-conformance-daily job that runs daily e2e tests of k8s-cluster-api-provider project using sonobuoy with mode conformance

Fixes #568 (closed)