Proper PmidNode Initialisation (max diskspace and account)

For PmidNode persona, disk space info/config need to be provided or dynamic calculated.
This may enforce a config file / option to be presented to vault executable.

There is another issue once address relocation (https://github.com/maidsafe/rfcs/pull/40) is introduced:
A pmid_node node is a non-client vault playing the role of storing immutable_data.
The current implementation assumes the network address of a pmid_node is it's associated account and will be stored on its correspondent PmidManger group.
This means the network address of pmid_node needs to be determined by account, which is conflict with the dynamic requirement for vault's other roles.

One possible solution is to make pmid_node a separate vault instance using pre-determined network address derived from account, meanwhile other personas kept in another vault instance using dynamic-determined network address.