We have several ways to specify the `rpc-endpoint`: with a flag,
with a single config file or multiple files. Before, the `rpc-endpoint`
flag was marked as required. Because `cobra` checked the required flag
presence first, it prevented specifying `rpc-endpoint` with a config file.
Signed-off-by: Aleksey Savchuk <a.savchuk@yadro.com>
For `frostfs-cli` it is ok to use grpc-client without blocking,
as `frostfs-cli` will perform RPC call anyway.
Signed-off-by: Dmitrii Stepanov <d.stepanov@yadro.com>
