Refactored DictField + Added value size limit to metadata field

[dn0]

Related update of user guide: erigones/esdc-docs@9182b99

[secult]

It doesn't work if you put an apache license as a value multiple times to the metadata

[dn0]

@secult In commit f7c9974 you can see some forgotten code, that is not needed anymore. The idea of the repr and load_repr was that we wanted to allow copy-paste of jsons into metadata values (which accept only strings). In the mean-time we've introduced raw json editing, which made this code obsolete. If you look at load_repr() you'll find a bug -> the string is modified, then parsed and in case of an error the modified string is returned (instead of the original string).

Obviously this is not the real problem. We are changing VM configuration via vmadm update and we pass the json with the update payload via stdin. vmadm calls fs.readFile('/dev/stdin') (https://github.com/erigones/smartos-live/blob/master/src/vm/sbin/vmadm.js#L697), which seems OK. But it turns out that fs.readFile is broken nodejs/node-v0.x-archive#7412 in nodejs shipped in SmartOS :(

TritonDataCenter/smartos-live#753