tempest: set compute-feature-enabled.volume_multiattach

This adds the ENABLE_VOLUME_MULTIATTACH flag and if True
configures Tempest to run volume multiattach tests.

Note that due to https://bugzilla.redhat.com/show_bug.cgi?id=1378242
we can't run multiattach tests with the Pike UCA packages since
those include qemu 2.10 and libvirt 3.6, and the valid versions for
multiattach support with libvirt is qemu<2.10 or libvirt>=3.10.

Depends-On: I80c20914c03d7371e798ca3567c37307a0d54aaa
Depends-On: I158c6f20e3e6a24bd2e5299abbeb3fc5208e5885

Part of nova blueprint multi-attach-volume

Change-Id: I46b7eabf6a28f230666f6933a087f73cb4408348
diff --git a/lib/tempest b/lib/tempest
index 35dbb7b..3b39dae 100644
--- a/lib/tempest
+++ b/lib/tempest
@@ -382,6 +382,10 @@
         fi
     fi
 
+    if [[ $ENABLE_VOLUME_MULTIATTACH == "True" ]]; then
+        iniset $TEMPEST_CONFIG compute-feature-enabled volume_multiattach True
+    fi
+
     if is_service_enabled n-novnc; then
         iniset $TEMPEST_CONFIG compute-feature-enabled vnc_console True
     fi