Add in logic for handling a field of 'data' to kv get

[jefferai]

We do this for normal commands in PrintRawField but it needs some help
for KV v2.

[jefferai]

Output; before it would error when attempting to get the data field:

$ vault kv put secret/foo zip=zap
Key              Value
---              -----
created_time     2018-07-10T16:48:26.942263352Z
deletion_time    n/a
destroyed        false
version          2

$ vault kv get -field=data secret/foo
map[zip:zap]

$ echo -n '{"data": {"data": { "foo": "bar" }}}' | vault write secret/data/bar -
Key              Value
---              -----
created_time     2018-07-10T16:48:45.459061705Z
deletion_time    n/a
destroyed        false
version          2

$ vault kv get -field=data secret/bar
map[foo:bar]