airskiff: Make airskiff gate non-voting

The Airskiff gate sometimes fails on irrelevant changes due to breaking
changes in other Airship projects. This stops safe changes from being
merged into treasuremap for extended periods of time. Several non-voting
checks have been proposed to other Airship components. Once they are
gating jobs, voting can be restored to the Airskiff job in treasuremap.

Change-Id: I79cbee671ee14e6712b5a2fefd236125a7e8107c

.zuul.yaml

@@ -96,6 +96,7 @@
     nodeset: airship-treasuremap-single-node
     description: |
       Deploy Memcached using Airskiff and latest Treasuremap changes.
+    voting: false
     timeout: 9600
     pre-run:
       - tools/gate/playbooks/git-config.yaml