Automatic "home directory"/~ style prefix access

[swhiteman]

In setting up a new role it would be great to specify a prefix that matches the username, for situations where you have many users and they each get their own implicit keyspace.

For example:

etcdctl role grant-permission myrolename --prefix=true read /~/

Then a user somebody has access to the keys with prefix /somebody/, etc.

Understanding the real-time interpolation of the username would cost a tiny bit.