Fix DISTRO_NAME usage for centos7 element

In each of the changed files in this patch, the $DISTRO_NAME usage
is checking against "centos".  The valid value for the centos7
element for $DISTRO_NAME is "centos7"[1].

[1] http://git.io/8IvuTw

Change-Id: I5d59e195c38fc7ff5b3235e195171fafcf5b4f87
Merge "Fix DISTRO_NAME usage for centos7 element"