Rename NodeInfo's publicKey and nodePublicKey

[fboucquez]

Regarding the names, I prefer names that are useful to the user, not based on where the property comes from.
For instance, I don't know if the user is familiar enough with the concept of Certificate Authority. Is this the key used for signing new blocks? Can we call it something like signingPublicKey or harvestingPublicKey?
Regarding nodePublicKey, this is also confusing, since all these keys are related to the node somehow. This is the key used to activate Delegated Harvesting, and it relates to TLS somehow, right? Can we call it something like communicationPublicKey, tlsPublicKey, transamissionPublicKey, networkPublicKey, ... ?

Originally posted by @segfaultxavi in #244 (comment)

[fboucquez]

About publicKey, nodePublicKey names. There are not good enough but we don't want to break too much.

The renames would need to be done in another PR until we have all the upgrades in place at least from server and rest.

nemtech/catapult-server#110

What would it be good candidates from them? It seems like they have multiple uses.

Maaaybe:

publicKey => caPublicKey / mainPublicKey = coming from ca.pem.key file, the node main account public key, and certificate authority.
nodePublicKey => nodePublicKey / communicationPublicKey = coming for node.pem.key file required for communication and delegate harvesting