lib, vtysh: vrf walkup bugfix

lib, vtysh: vrf walkup bugfix

Static route commands are now installed inside the VRF nodes. This has
quietly broken top-level static routes in certain scenarios due to
walkup logic resolving a static route configuration command inside
VRF_NODE first if the command is issued while in a CLI node lower than
VRF_NODE. To fix this VRF_NODE needs a special exit command, as has been
done for many other nodes with the same issue, to explicitly change the
vrf context to the default VRF so that when walkup resolves against the
VRF node it will configure against the default VRF as desired.

Of course this is a hack on top of a hack and the CLI walkup
implementation needs to be rewritten.

Signed-off-by: Quentin Young <qlyoung@cumulusnetworks.com>