add icon to Helm chart

[smiklosovic]

There are icon fields missing in Chart.yaml for both charts and helm lint says this:

[smiklosovic@E091-FED helm]$ helm lint cassandra
==> Linting cassandra
[INFO] Chart.yaml: icon is recommended

1 chart(s) linted, no failures
[smiklosovic@E091-FED helm]$ helm lint cassandra-operator
==> Linting cassandra-operator
[INFO] Chart.yaml: icon is recommended

1 chart(s) linted, no failures

It would be nice to have that icon in place.

The thing is that we can not host easily an SVG because of security reasons (@zegelin ping me if you want to know more) we will go with PNG for now.

Possible icons given to me are:

Icons:

https://www.instaclustr.com/wp-content/uploads/2019/09/Instaclustr-Icon-C-whitecyan.png
https://www.instaclustr.com/wp-content/uploads/2019/09/Instaclustr-Icon-C-navycyan.png

Cyan and Navy logo
https://www.instaclustr.com/wp-content/uploads/2019/09/Instaclustr-C-cyannavy-640x360.png
grey and Navy logo
https://www.instaclustr.com/wp-content/uploads/2019/09/Instaclustr-C-greynavy-640x360.png
Navy and Cyan logo
https://www.instaclustr.com/wp-content/uploads/2019/09/Instaclustr-C-navycyan-640x360.png
Navy and Grey logo
https://www.instaclustr.com/wp-content/uploads/2019/09/Instaclustr-C-navygrey-640x360.png
White and Cyan logo
https://www.instaclustr.com/wp-content/uploads/2019/09/Instaclustr-C-whitecyan-640x360.png
White and Grey logo
https://www.instaclustr.com/wp-content/uploads/2019/09/Instaclustr-C-whitegrey-640x360.png
White and Navy logo
https://www.instaclustr.com/wp-content/uploads/2019/09/Instaclustr-C-whitenavy-640x360.png

It is hard to say for now what colour combination we go with as it depends in what context that icon is displayed but it was said to me the most commonly used are 'cyan and navy' and "navy and cyan".