Consider adding pull secret to custom service accounts

[idlewis]

If a pull secret is specified in the CR, it is currently added to the service account used to run the liberty application.

If a custom service account is specified in the CR, the pull secret is not added.

Consider adding a mechanism to enable pull secrets when a custom service account is used. This is in response to issue #646

[kabicin]

Hi @leochr and @idlewis, as discussed in standup, we'll document the limitation for now, thanks. I've opened #681.

[leochr]

@kabicin Merged #681. Please open a doc issue to update WLO docs with the same. Thank you.

[kabicin]

Looks like I missed this notification on my list so just getting back to it. I've opened https://github.ibm.com/websphere/liberty-docs/issues/4320 for a mirrored update.