Handle connection failures due to server load

SSHException with "Error reading SSH protocol banner" is raised when
sshd is unresponsive but Fabric treats it as an auth failure. This patch
fixes this by treating it as NetworkError.
Add a changelog entry for #948
Merge branch '1.8' into 948-int

Conflicts:
	docs/changelog.rst
Merge branch '1.8'

Conflicts:
	docs/changelog.rst