etcd: Connection options (socket file, ipv6)

- Allow unix domain socket to be used as client url
- Using ::1 as clienturl should not create default local ipv4 listener
- Add shell tests

test/test-gotest.sh

@@ -21,7 +21,7 @@ fi
 
 # As per https://github.com/travis-ci/docs-travis-ci-com/blob/master/user/docker.md
 # Docker is not supported on Travis macOS test instances.
-if [[ "$TRAVIS_OS_NAME" == "osx" ]];then
+if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then
 	XTAGS+=('nodocker')
 fi