boltzcli: finish up entity commands

[kilrau]

• missing remove, should only allow to remove if no wallet associated with this entity
• Descriptions wrong

   Entity:
     entity  Manage the wallets used by the client

DESCRIPTION:
   Creates a new wallet for the specified currency and unique name.
   Currency has to be BTC or LBTC (case insensitive).

• boltzcli wallet create doesn't allow me to say which entity this wallet should belong to... which is the one thing about entities I expected I can control via boltzcli

$ boltzcli wallet create help
NAME:
   boltzcli wallet create - Create a new wallet

USAGE:
   boltzcli wallet create [command options] name currency

DESCRIPTION:
   Creates a new wallet for the specified currency and unique name.
   Currency has to be BTC or LBTC (case insensitive).

OPTIONS:
   --help, -h  show help