Role Templates should have defaults

[russjones]

Problem

At the moment, role templates don't have defaults. That means you have to explicitly define resources, namespaces, node labels and the session TTL in the template. This creates a bad UX and potentially security issues because it leads to thing like:

resources:
   "*": ["read", "write"]

Potential Solution

Role templates should use default values like local users do: https://github.com/gravitational/teleport/blob/master/lib/services/role.go#L43-L66