Document specifying secrets for CHAP authentication

[Patricol]

To save the next person some time - as best I can tell from the code - you add a volumeAttribute (to the pv) named 'secret'; with a value matching this pseudocode format:

jsonencode({
  "node.session.auth.username": "your userid"
  "node.session.auth.password": "your password"
  "node.session.auth.username_in": "your mutual_userid"
  "node.session.auth.password_in": "your mutual_password"
  "node.sendtargets.auth.username": "your discovery userid"
  "node.sendtargets.auth.password": "your discovery password"
  "node.sendtargets.auth.username_in": "your discovery mutual_userid"
  "node.sendtargets.auth.password_in": "your discovery mutual_password"
})

[Patricol]

Actually you can specify with kubernetes secrets; put the content described above in a secret and use ControllerExpandSecretRef/ControllerPublishSecretRef/NodePublishSecretRef/NodeStageSecretRef to point to it. (Not sure which are actually needed; no harm in giving all four afaict.)

[humblec]

Please feel free to send a PR @Patricol

[vamage]

/assign vamage going to add this to docs based on open-iscsi