Fix key.meta.vault for root dir keys && read vault.meta without vault key

[svyatonik]

Details:

1. When key is moved from vault back to the root directory, it's meta still contains the vault field. Fixed + added test on RPC level
2. As vault meta is intended to contain hint for vault password, it was a bad idea to allow reading meta for opened vaults only => added support for reading unopened vaults meta + updated test.

[svyatonik]

@jacogr

[svyatonik]

3. Added support for vaults, created before vault.meta appeared (since it is 'no cost' && it fails if there is no meta field in vault files)

[rphmeier]

Did these "old vaults" exist in any published release? Why bother supporting them if not?

[svyatonik]

@rphmeier Nope, but meta field for vaults is actually optional (this is also true for key files: KeyFile works the same way:
https://github.com/ethcore/parity/blob/master/ethstore/src/json/key_file.rs#L101 ). So it seems logical to me to react as return meta=None if there's "meta": null in json, than to fail RPC request.