RD auto-assigment based on roque Suggestion

[sbarguil]

No description provided.

[boucadair]

@oscargdd: Please link this PR to #114 so that issue is closed automatically when this PR is merged. Thanks.

[oscargdd]

In the auto-asignment cases add a read-only leaf with the asigned value, so it can be retrieved

[oscargdd]

Proposal:
choice rd-choice {
case directly-assigned {
leaf rd {
type rt-types:route-distinguisher;
}
}
case auto-assigned {
leaf auto-rd {
type empty;
}
read only leaf rd-asigned {
type rt-types:route-distinguisher;
config false;
}
}
case pool-assigned {
leaf rd-pool-name {
type string;
}
leaf rd-asigned {
type rt-types:route-distinguisher;
config false;
}
}
case no-rd {
leaf no-rd {
type empty;
}
}
}