[pgAdmin4][Patch] Feature #1447 SSH Tunnel

I have implemented the SSH Tunnel support using https://pypi.org/project/sshtunnel/ python package. Added "SSH Tunnel" Tab in server dialog. This implementation supports user name /password and private/public key combination with Passphrase to crate SSH Tunnel. I have added regression test case to add server using SSH Tunnel options.

The given python package(https://pypi.org/project/sshtunnel/) support Python version 2.7, 3.4+.

It uses Paramiko (Python implementation of SSHv2 protocol) which actually drops support for Python 2.6. So I have added SUPPORT_SSH_TUNNEL parameter in config.py which checks the python version and set the flag accordingly. In case of Python 2.6, 3.0, 3.1, 3.2 and 3.3 control on the "SSH Tunnel" tab of server dialog will be disabled.

Please review it, and if looks good please commit the code.     
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