3scale-operator operator-sdk upgrade to latest version (v0.15.2)

We should upgrade the operator-sdk version used by the 3scale-operator from the current version used (v0.8.0) to the latest available version (at the moment of writing this v0.15.2).

The motivations for this are:

• Integr8ly team wants some metrics functionalities provided by newer versions of the operator-sdk
• Have new improvements and functionalities that are provided by operator-sdk newer versions
• Improvements and fixes in OpenAPI validation specifications
• Upgrade of Go version to 1.13 being integrated with modules
• Improvements in OLM functionalities, CSV generation and manifests management
• Updates on the kubernetes API, controller-runtime library and other major libraries
• Lots of other new changes and functionalities. See https://github.com/operator-framework/operator-sdk/blob/master/CHANGELOG.md

Breaking changes are introduced in new operator-sdk versions so we have to take precautions with that when upgrading.