Fixed conditions to determine if current distribution is a Fedora based distribution

stack.sh line 223:
if [[ is_fedora && $DISTRO == "rhel6" ]]; then

stack.sh line 234:
if [[ is_fedora && ( $DISTRO == "rhel6" || $DISTRO == "rhel7" ) ]]; then

stack.sh line 358:
if [[ is_fedora && $DISTRO == "rhel6" ]]; then

Condition [[ is_fedora && $DISTRO == "rhel6" ]] return wrong result.
This condition is equivalent to the
[[ -n is_fedora && $DISTRO == "rhel6" ]]. First expression -n is_fedora
always not null, therefore this condition the same is
[[ $DISTRO = "rhel6" ]].

Change-Id: Ida9eaa7950554bcd2f183dede7ad19522f9ca558
Closes-Bug: #1393684
Merge "Fixed conditions to determine if current distribution is a Fedora based distribution"