cli: add 'Default:' to help test

[troglobit]

The 'help' command, added by KernelKit for use in Infix, formats the YANG model description text and prints it to stdout. A great imrovement would be if this could also print any default value.

Example

Currently:

root@infix-00-00-00:/config/interface/docker0/container-network/> help subnet 
NAME
        subnet <union>

DESCRIPTION
        Subnet to assign addresses from, round-robin assignment.
        The default is from a standard Docker setup.

Proposal:

root@infix-00-00-00:/config/interface/docker0/container-network/> help subnet 
NAME
        subnet <union>

DESCRIPTION
        Subnet to assign addresses from, round-robin assignment.
        The default is from a standard Docker setup.

DEFAULT
        172.17.0.0/16

Side Note

Although technically correct, <union> is not really helpful. In this it would be more helpful, I think, to print <inet:ip-prefix>, or simply <ip-prefix>.