Fix for RHEL6

The correct workflow for building RHEL6 images includes not using
the rhel-registration scripts in this element, but rather the 00-rhsm
script.  This patch updates the return value from 1 to 0 for the case
when the REG_METHOD is left unset.  This will allow the RHEL6 images
to build without needing to set REG_METHOD. This patch also improves
the note about RHEL6 image building.

This patch is the result of the discussion in the comments of [1].

[1] Iff7b9fc30d5a36231598a977a9edcd55229766c5

Change-Id: I2f35b8d7d8749d44d88f06e9e2c3116ff93b88fe
Closes-Bug: 1404364

elements/rhel-common/README.md

@@ -14,6 +14,7 @@ IMPORTANT NOTE:
 The 00-rhsm script is specific to RHEL6.  If you use the REG_ variables to
 use with RHEL7, you do not need to set any DIB_RHSM variables.  The scripts
 named with "rhel-registration" have not been developed or tested for RHEL6.
+For information on building RHEL6 images, please see the rhel element README.
 
 Environment Variables For Image Creation
 ----------------------------------------

elements/rhel-common/pre-install.d/00-rhel-registration

@@ -103,5 +103,5 @@ case "${REG_METHOD:-}" in
         ;;
     *)
         echo "WARNING: only 'portal', 'satellite', and 'disable' are valid values for REG_METHOD."
-        exit 1
+        exit 0
 esac