Don't hard-code provider gateway address

[boz]

This should be a variable:

https://github.com/ovrclk/helm-charts/blob/b52f46016de4a4516c6a4711fecead0cb58d7bbf/charts/akash-provider/templates/configmap-boot.yaml#L64

[sacreman]

We got the docs to the point where we tell people to "create these dns entries under the domain you want to use". I agree this could be a variable but then it adds extra complication to the docs.

[arno01]

We can add a variable which, if not set, would default to what we already have provider.{{ .Values.domain }}.
This way users can easily change "provider." by passing the helm variables, instead of having to change a hardcoded "provider." bit.

And we can mention in the docs just one line:
--set dns.provider=provider.<domain>