Improve infrastructure role definitions (#208)

Enhance definitions of infrastructure roles by allowing membership in multiple roles, role options and per-role configuration to be specified in the infrastructure role configmap, which must have the same name as the infrastructure role secret. See manifests/infrastructure-roles-configmap.yaml for the examples and updated README for the description of different types of database roles supposed by the operator and their purposes.

Change the logic of merging infrastructure roles with the manifest roles when they have the same name, to return the infrastructure role unchanged instead of merging. Previously, we used to propagate flags from the manifest role to the resulting infrastructure one, as there were no way to define flags for the infrastructure role; however, this is not the case anymore.

Code review and tests by @erthalion