lib: allow 'do' commands in ENABLE_NODE

lib: allow 'do' commands in ENABLE_NODE

'do' is syntax sugar that allows the user to execute a command under
ENABLE_NODE when in another CLI node. If the user is already in
ENABLE_NODE, use of 'do' was previously disallowed. This patch allows it
because it makes it easier for us to hack around certain instances of
the node synchronization problem with vtysh.

Also included is a fix for one of these problems.

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