Add small /srv volume for optional swift data

Most tripleo node roles store their state in /var with the exception
of the ObjectStorage role which stores its state in /srv.

This change adds a very small /srv volume which will generally remain
empty.  A documentation change will be proposed which shows how to set
custom growvols values to grow /srv in the rare case that the
ObjectStorage role is used.

Change-Id: Ide8776ffdb62bff46c3df4773fc7b01e9710373f
Blueprint: whole-disk-default
(cherry picked from commit 998936c734)

elements/overcloud-partition-uefi/block-device-default.yaml

@@ -46,10 +46,13 @@
           extents: 4%VG
         - name: lv_audit
           base: vg
-          extents: 4%VG
+          extents: 3%VG
         - name: lv_home
           base: vg
           extents: 4%VG
+        - name: lv_srv
+          base: vg
+          extents: 1%VG
 - mkfs:
     name: fs_root
     base: lv_root
@@ -105,4 +108,13 @@
         fstab:
             options: "rw,nodev,relatime"
             fsck-passno: 2
+- mkfs:
+    name: fs_srv
+    base: lv_srv
+    type: xfs
+    mount:
+        mount_point: /srv
+        fstab:
+            options: "rw,nodev,relatime"
+            fsck-passno: 2