Use 'pipe' variable format instead of default
Previously, when user selected multiple variables, this query
```
service ~= "$service"
```
was expanded into
```
service ~= "{svc1, svc2}"
```
After the change, it will be turned correctly into
```
service ~= "svc1|svc2"
```
We use `pipe` instead of `regex` because there seems to be a bug in
riemann that will make the following fail
```
host ~= "host1\.example\.com|host2\.example\.com"
```
pipe doesn't backslash the dot but this is a workaround
i