tests: increase memory to 1024Mb for centos7_cluster scenario

tests: increase memory to 1024Mb for centos7_cluster scenario

we see more and more failure like `fatal: [mon0]: UNREACHABLE! => {}` in
`centos7_cluster` scenario, Since we have 30Gb RAM on hypervisors, we
can give monitors a bit more RAM. By the way, nodes on containerized cluster
testing scenario have already 1024Mb memory allocated.

Signed-off-by: Guillaume Abrioux <gabrioux@redhat.com>
(cherry picked from commit bbb869133563c3b0ddd0388727b894306b6b8b26)
Signed-off-by: Guillaume Abrioux <gabrioux@redhat.com>