Rename roles to follow a readable schema

Give all roles a "float-" prefix to avoid collisions with user-defined
roles.

Separate Ansible roles into three main categories:

- *base* for the low-level mechanics of getting the machines ready
  to host services, provide service discovery, credentials, and other
  such mechanisms.

- *infra* for the configuration of services that are part of the
  float infrastructure but run on the base layer themselves

- *util* for internal utility roles