Use JAVACMD rather than path-default java to check Java version

The cdap-standalone *nix startup script (`cdap.sh`) determines the `java` command to use based on `JAVA_HOME`, but uses the default `java` command on the path to check that an appropriate Java version is in use.  If the default `java` on the path is Java 8+, then the script fails with a message that the Java version is not 6 or 7, even if JAVA_HOME points to a 6/7 installation.