zuul: Update Zuul nodeset name

This commit seeks to standardize the Zuul nodeset name to match other
projects and the existing `ubuntu-xenial` label.

Change-Id: I199e47da2a3738388aea612e43c0f55ed8bf2366

.zuul.yaml

@@ -27,7 +27,7 @@
 - nodeset:
     name: airship-treasuremap-single-node
     nodes:
-      - name: primary
+      - name: ubuntu-xenial
         label: ubuntu-xenial
 
 - job:

tools/gate/playbooks/airship-seaworthy-site-lint.yaml

@@ -12,7 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-- hosts: primary
+- hosts: ubuntu-xenial
   tasks:
     - name: Lint airship-seaworthy site
       shell: |

tools/gate/playbooks/airskiff-site-lint.yaml

@@ -12,7 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-- hosts: primary
+- hosts: ubuntu-xenial
   tasks:
     - name: Lint airskiff site
       shell: |

tools/gate/playbooks/git-config.yaml

@@ -12,7 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-- hosts: primary
+- hosts: ubuntu-xenial
   tasks:
     - name: Git config
       shell: |

tools/gate/playbooks/install-docker.yaml

@@ -12,7 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-- hosts: primary
+- hosts: ubuntu-xenial
   tasks:
     - name: Install Docker CE
       shell: |