Lookup for keystore in a testnet dir

[sammy007]

I believe parity should look for accounts in ~/Library/Ethereum/testnet/keystore if --testnet used.

[gavofyork]

@NikVolf this is more short-sighted geth-compatibility than core client stuff.

for this to work nicely, we need to separate the keystore path (e.g. --keys-path) from the main DB path, then as a geth-compatibility thing, make --testnet mean two things; --chain morden (it means this already) and --keys-path $ETHEREUM/testnet/keystore (where $ETHEREUM = $HOME/.ethereum on linux and $HOME/Library/Ethereum on Mac).

[jorisbontje]

Running parity --chain morden --keys-path ~/Library/Ethereum/testnet/keystore causes parity to dump my mainnet geth keys into the testnet keystore; while ignoring my testnet keys. So that doesn't work as viable workaround.

[gavofyork]

indeed - also need to avoid parity importing anything when a non-standard keys-path is provided. #1112