(maint) Switch to new el5 image for acceptance tests

The old image had CentosPlus enabled, which pulled in an odd postfix version
that pulled in a conflicting 'postgresql' package. This new image no longer
uses the CentosPlus repository, since we weren't using it before this anyway
it probably won't matter.

Signed-off-by: Ken Barber <ken@bob.sh>
Merge pull request #1171 from kbarber/maint/stable/fix-postfix-postgresql-dep-acceptance-failures

(maint) Switch to new el5 image for acceptance tests