Add OSPF stub area configuration options

[stmosher]

This PR allows for the configuration of OSPF area types of stub, totally-stubby, and not-so-stubby (nssa) and their associated parameters.

For instance, the below would configure area "1" as an OSPF stub area. ospfv2: areas: area: - config: identifier: 1 identifier: 1 stub-options: stub: config: enabled: True default-information-originate: True totally-stubby: config: enabled: False default-information-originate: False nssa: config: enabled: False no-summary: False default-information-originate: False I believe these are configuration that users of OSPFv2 need. Please let me know if you have any questions or need anything else.

[OpenConfigBot]

Compatibility Report for commit de54d556343c16bb4f52484d39b91101e75db582: ⛔ yanglint@SO 1.10.17

[stmosher]

This will solve issue: https://github.com/openconfig/public/issues/530