increase stability of the existing ssh command

[tedteng]

Describe the bug
gardenctl ssh not stability when connecting AWS. most of time exit with an error

kex_exchange_identification: Connection closed by remote host
exit status 255

To Reproduce
gardenctl ssh nodename

Expected behavior

Screenshots

Gardenctl Version (please complete the following information):
gardenctl:
version : v0.18.0
build date : 2020-07-13
go version : go1.14.3
go compiler : gc
platform : darwin/amd64

Additional context
Add any other context about the problem here.

[tedteng]

/assign