ssh: parse ~/.ssh/config to get hostname, username, port and key
ssh: use ProxyCommand from ~/.ssh/config as transport

This allows ncclient to contact a router through another machine by
parsing ProxyCommand from ~/.ssh/config.
ssh: don't use port from ~/.ssh/config

Even when the port in ~/.ssh/config is not present, Paramiko will return
22, overriding our default (830).
Merge branch 'feature/proxycommand' of https://github.com/vincentbernat/ncclient into vincentbernat-feature/proxycommand
Minor wording fix in ssh_config param doc
Merge branch 'vincentbernat-feature/proxycommand'