Use Travis's tag env var for latest calculation

Signed-off-by: Nolan Brubaker <brubakern@vmware.com>

hack/gcr-push.sh

 #!/bin/sh
 
 # Return value is written into LATEST
-LATEST=""
-function latest_release() {
-# Loop through the tags since pre-release versions come before the actual versions.
-# Iterate til we find the first non-pre-release
-    for t in $(git tag -l --sort=-v:refname);
-    do
-        # If the tag has alpha, beta or rc in it, it's not "latest"
-        if [[ "$t" == *"beta"* || "$t" == *"alpha"* || "$t" == *"rc"* ]]; then
-            continue
-        fi
-        LATEST="$t"
-        break
-    done
+LATEST=false
+function is_latest_release() {
+    # If the tag has alpha, beta or rc in it, it's not "latest"
+    if [[ "$TRAVIS_TAG" == *"beta"* || "$TRAVIS_TAG" == *"alpha"* || "$TRAVIS_TAG" == *"rc"* ]]; then
+        LATEST=false
+    else
+        LATEST=true
+    fi
 }
 
-
 # Always publish for master
 if [ "$BRANCH" == "master" ]; then
     VERSION="$BRANCH" make all-containers all-push
@@ -24,14 +18,8 @@ fi
 
 # Publish when TRAVIS_TAG is defined.
 if [ ! -z "$TRAVIS_TAG" ]; then
-    # Calculate the latest release
-    latest_release
-
-    # Default to pre-release
-    TAG_LATEST=false
-    if [[ "$TRAVIS_TAG" == "$LATEST" ]]; then
-        TAG_LATEST=true
-    fi
+    # Check if this is the latest release.
+    is_latest_release
 
-    VERSION="$TRAVIS_TAG" TAG_LATEST="$TAG_LATEST" make all-containers all-push
+    VERSION="$TRAVIS_TAG" TAG_LATEST="$LATEST" make all-containers all-push
 fi