cli: Use import instead of create to open Ledger account

[matevz]

Term "create" should be used if the CLI generates a new keypair. When adding a Ledger account into the wallet, CLI doesn't generate or overwrite anything, but just registers existing Ledger account to the CLI.

Use oasis wallet import to add the Ledger account instead of oasis wallet create. In this case importing would also be done interactively so the user can see the Ledger option and easily provide the key derivation index (no need for non-obvious --kind ledger and --number x flags). This also makes the CLI flow consistent with Oasis wallet web and extension.