Refactor `put_config_dict` to accept data for multiple models

[wkz]

E.g. instead of...

target.put_config_dict("ietf-system", { "system": { "hostname": "asdf" } })
target.put_config_dict("ietf-interfaces", { "interfaces": { "interface": [{"name": "eth0", "enabled": False }] } })

...we want either....

target.put_config_dict({
    "ietf-system": { "system": { "hostname": "asdf" } },
    "ietf-interfaces": { "interfaces": { "interface": [{"name": "eth0", "enabled": False }] } },
})

...or perhaps....

target.put_config_dict({
    "ietf-system:system": { "hostname": "asdf" },
    "ietf-interfaces:interfaces": { "interface": [{"name": "eth0", "enabled": False }] },
})