remote: ADD sock attribute and pass it to SSHConnection constructor (#3562)
remote: ADD sock attribute and pass it to SSHConnection constructor (#3562)

* remote: ADD sock attribute and pass it to SSHConnection constructor

The sock attribute is set to a `paramiko.ProxyCommand` object
if 'ProxyCommand' exists in config to `None` other-wise
(which is the default option in `paramiko.client.SSHClient.connect()`).

FIXES #3560

* Move paramiko import to RemoteSSH __init__

* Update dvc/remote/ssh/__init__.py

* Update dvc/remote/ssh/__init__.py

* Update dvc/remote/ssh/__init__.py

* Update dvc/remote/ssh/__init__.py

Co-authored-by: Sergio Garcia <83ed8eda10d28341cab92990bffbb30447b34520@zhilabs.com>
Co-authored-by: Ruslan Kuprieiev <f6538312ce47017d734b1357aabc970ebf896e32@gmail.com>
efiop/dvc,dmpetrov/dataversioncontrol,dmpetrov/dataversioncontrol,efiop/dvc