Increase ram for tempest tests (aarch64)

Increase ram for tempest tests (aarch64)

The following tests fail on armband because the VM cannot boot.
tempest.api.compute.servers.test_server_actions.
ServerActionsTestJSON.test_reboot_server_hard

tempest.scenario.test_network_basic_ops.
TestNetworkBasicOps.test_network_basic_ops

tempest.scenario.test_volume_boot_pattern.
TestVolumeBootPattern.test_volume_boot_pattern

tempest.scenario.test_volume_boot_pattern.
TestVolumeBootPatternV2.test_volume_boot_pattern

The reason for the failure is that the 128 ram is insufficient for the
image to boot. The fix consists in increasing the ram to 256

JIRA: ARMBAND-61

Change-Id: I5be1a44a8038b4952cca100511e8d08b7a37681c
Signed-off-by: Cristina Pauna <cristina.pauna@enea.com>
(cherry picked from commit 5a417f076549941e0dae176849045aab32bf0be5)