More for DOC-1431: refining credential and eucarc instructions; adding links to instance tasks.
Fixes for DOC-1353: removing mentions of SYSTEM and STATIC networking modes from the Install Guide.
Merge branch '4.0'

* 4.0:
  Fixes for DOC-1353: removing mentions of SYSTEM and STATIC networking modes from the Install Guide.
  More for DOC-1431: refining credential and eucarc instructions; adding links to instance tasks.