More complete validation of endpoint for gobgp grpc

[kurojishi]

as for #29

This validates a bit more thoroughly while not expecting an IP address (and so not validating that), but uses existing checks for url validation and keeps a bit more schemas that are valid schema against grpc endpoints.

(it may allow in time to even use https encryption if the gobgp daemon is farther away)

[kurojishi]

hi, can anyone give it an check @greenpau