lcd/client: Abstract method for determining when to get passphrase from stdin

[ValarDragon]

Ref: #931 (comment)
Basically change that part of the code to not depend on the hardcoded string "local", and have it be some property of that object. This was moved to an issue so that the ledger PR can be merged in.

[alexanderbez]

Addressing this in #1741

[alexanderbez]

@ValarDragon I've already made that logic enumerated, but having that be part of the type (I call it KeyType) might be difficult because of import cycles. Did you have other thoughts on this?

Right now it's more or less:

// we only need a passphrase for locally stored keys
if keyInfo.GetType() == cryptokeys.TypeLocal {
    passphrase, err = GetPassphraseFromStdin(name)
    if err != nil {
        return fmt.Errorf("Error reading passphrase: %v", err)
    }
}

[jackzampolin]

Closing this as implemented.