PYNOS-93: Add support for loopback interfaces
PYNOS-93: Add support for loopback interfaces

Changes:

+ Add support for loopback to `Interface.ip_address`
+ Add support for loopback to `Interface.admin_state`

Caveats: None

Test Plan:

* `tox`

Change-Id: I39b89802d6979f00b16df12064dcabed9eab5ad6

BRCDcomm/pynos,brocade/pynos