Allow configuring fission namespace

[soamvasani]

fission-ui hardcodes the controller namespace as "fission". However with the fission helm chart, the namespace could be anything that the user sets; when it's not "fission", fission-ui nginx crashes with:

2017/08/10 00:35:15 [emerg] 1#1: host not found in upstream "controller.fission" in /etc/nginx/conf.d/default.conf:36
nginx: [emerg] host not found in upstream "controller.fission" in /etc/nginx/conf.d/default.conf:36

This seems to be the place it's hardcoded:
https://github.com/fission/fission-ui/blob/master/docker/fission-ui.conf#L36

Some sort of environment variable would do the trick. Then we could set it in the deployment yaml.

[yqf3139]

@soamvasani
I just make the change. And I change the tag to latest.
I will have a look at fission helm chart config for ui later.

[yqf3139]

And there is an other solution to use just controller as upstream, but for flexibility, in current solution ui can deploy in another namespace.