Support for imagePullSecrets

[brettcurtis]

I'm brand new to this so hopefully I didn't miss anything. This should address part of #34.

[chadlwilson]

Thanks for the contribution @brettcurtis !

Made some tweaks to make this releaseable

• changed the structure to be similar to the Bitnami charts, i.e server.image.pullSecrets, agent.image.pullSecrets
• allow arrays to be supplied. This might be relevant if people are using initContainers and multiple registries
• extend to the server and tests images so the chart is consistent
• bumped version, updated docs etc. We should probably have a PR template for this stuff to remind folks (including myself!). I'll try and add one, probably inspired by something like Bitnami use, e.g https://github.com/bitnami/charts/pulls

Will be released in 1.40.1 shortly.