xud master password integration

[kilrau]

Now that we pushed back the deadline, let's go for the integration of ExchangeUnion/xud#1006. It'd be good to have some people test this on testnet.

After selecting testnet/mainnet - check if lndbtc wallet exists in ~/.xud-docker/testnet / ~/.xud-docker/mainnet (we'll make lndbtc a must in all later setups since lndbtc is required anyways to generate the seed).

If wallet does not exist:

• No existing wallets found. Creating wallets...
• Call CreateNode, Please set a password (it secures xud and all your wallets):
• print potential errors and exit
• If no errors, continue with Please write down this 24 word mnemonic. You will be able to recover xud and funds of all your wallets with it, in case you forget your master password or loose your device. Keep it somewhere safe, it's your ONLY backup: CreateNode output
• Did you write down your 24 word mnemonic? You will not be able to view it again after this prompt. [Y/n] n -> show again
• Y -> All wallets successfully created.
• start

If lndbtc wallet exists:

• call unlock, Please enter your xud password:
• start