(feat/monitor): Make ALLOW_SIGN_UP configurable via the chart

[lachie83]

This is to toggle the signup page on the grafana login screen

https://github.com/deis/monitor/blob/master/grafana/rootfs/usr/share/grafana/grafana.ini.tpl#L150

[lachie83]

I will work on a PR for this

[Cryptophobia]

This issue was moved to teamhephy/monitor#8