[op-mode] T1621 rewrite misc commands to python/xml syntax

[alkersan]

Please review another portion of 'trivial' rewrites from vyatta-op. Also see the corresponding PR in vyatta-op.

Notes and questions:

• monitor bandwidth-test initiate | accept rewritten as is
• generate ssh-server-key was rewritten in simple python script
• show history rewritten as is
• show host os rewritten as is
• show host date rewritten as is
• set terminal <misc opt> rewritten as is
• format disk like rewritten into python and merged with show disk definitions
• set <tag> was moved to set builtin <tag>, as top-level tagNodes aren't allowed

How this was tested:

• A QEMU VM image was built from the rolling vyos-1.2-rolling-201908301331-amd64.iso
• each rewritten command was executed manually after vyos-1x and vyatta-op deb packages re-install