Added ability to disable base_url in Helm chart

[nicknezis]

This PR adds a new config item to support disabling the base_url parameter when installing Heron using the Helm chart.

[nicknezis]

Closing this PR because the desired behavior can be attained by setting .Values.heron.url to /.

[nicknezis]

I tested setting .Values.heron.url to / and things didn't work. I believe this PR is still needed to use Heron UI with k8s NodePorts.

[Code0x58]

does setting --base_url= work as expected as I feel it should? If so, I'm thinking it would be cleaner to use something better than default when setting the url, and to instead use something like value = config_value if config_value is not None else "" - that would avoid the new coupled variable currently introduced by the PR.

[nicknezis]

@Code0x58 The issue I ran in when trying something similar to what you suggested, was that "" and null both resolved to a type that was not a String. So my attempt to add a comparison in the template failed. I'm not sure how to check type and type convert in the Helm template. Any suggestions?

[windhamwong]

Looks good with the OR comparison to use the default value.

need to solve the issue of default value

[windhamwong]

Masterminds/sprig#53 (comment)
This could be an insight of checking null or "".
Maybe checking if .Values.heron.url is empty or not? (eq nil?)

Helm syntax has no ability to do code = 'True' if True is True else 'False'.

[nicknezis]

I updated the logic to use more features I learned about Sprig Functions.

This latest version will use the single variable. If set to "-" it will use the full k8s proxy URL. I've made this the default. If a user sets the heron.url to any string, it will set that as the value. This includes the empty string case "". The third option that I needed was the ability to disable the base_url parameter. This can now be achieved by setting heron.url: null.