init_site.sh: sed in managementConfiguration

`airshipctl config` doesn't yet support setting a context's
managementConfiguration value.  This change adds in a sed
to set all managementConfigurations to default.

Change-Id: I0d8d0fb285a0b82e348640ef689eb3f0efb9e547
(cherry picked from commit bbd0ca2895)

tools/init_site.sh

@@ -103,3 +103,6 @@ airshipctl config set-manifest "${SITE}" \
 airshipctl config set-context ephemeral-cluster --manifest "${SITE}"
 airshipctl config set-context target-cluster --manifest "${SITE}"
 airshipctl config use-context ephemeral-cluster
+
+# TODO: use airshipctl for this once `airshipctl config` supports it
+sed -i "s|^    managementConfiguration:.*|    managementConfiguration: default|g" ~/.airship/config