Validate configured -alertmanager.web.external-url and fail if ends with /#4081

pracucci merged 3 commits intocortexproject:mastercortexproject/cortex:masterfrom

pracucci:fix-alertmanager-local-dev-and-config-validationpracucci/cortex:fix-alertmanager-local-dev-and-config-validationCopy head branch name to clipboard