Added support of lnd seed phrase in the pod config.

[randomlogin]

It's possible to set the lnd seed mnemonic in the pod config in the following way:

cipherSeedMnemonic: [above, rookie, pioneer, pen, truck, board, spatial, pond, destroy, civil, vote, zebra, coral, love, height, price, nephew, better, bar, wife, chaos, clock, pottery, rifle]

If this field is absent, it fallbacks to seed generation via rpc.

For (any) backward compatibility, if the seed is provided in the config, it is also written to /tmp/genseed.json (thought without encrypted field).

[pinheadmz]

I dont mind also writing the phrase to the data dir in plain text, but I think it should still be generated randomly when the pod starts. I also don't totally understand the use case yet. What do you need it for?