docs: Add another way to view the Synnefo UI

Add instructions on how to connect to the Synnefo UI using a SOCKS
proxy.
deploy: Remove obsolete UI_MEDIA_URL setting

Deploy used to set UI_MEDIA_URL to point to the IP of the host
instead of its FQDN, to work around issues with the Java-based VNC
client and self-signed certificates. Synnefo v0.16 has obsoleted
the Java VNC console, so we may safely remove this setting.
astakosclient: Create exception 'ConnectionError'

When astakosclient fails to connect to the remote server, raise a
'ConnectionError'. This exception will contain the original Exception
thrown by httplib so that can be used by other clients such as kamaki.