[pgAdmin4][Patch] Feature #1447 SSH Tunnel

Hi Hackers

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.

--
*Akshay Joshi*

*Sr. Software Architect *

*Phone: +91 20-3058-9517Mobile: +91 976-788-8246*