correcting the elements sequence for centos+hdp+plain+debug

When creating a centos image with the hdp plain plugin and with debug
mode on the root password is not set correctly. This change fixes the
element sequence.

Changes
* renaming the elements sequence variable for centos hdp plain images

Change-Id: I9e6a46c64cf6239fd6635c2e5673b258a0c4a6fd
Merge "correcting the elements sequence for centos+hdp+plain+debug"