ssh: add missing import, fix typo

************* Module salt.ssh
E1101:439,18:prep_trans_tar: Module 'json' has no 'dumpd' member
E0602:454,4:prep_trans_tar: Undefined variable 'shutil'
ssh: add another missing import (`copy` std lib module)
Merge pull request #6478 from cvrebert/patch-2

ssh: add missing import, fix typo