TACHYON-63 Remove script error "[: too many arguments" in tachyon-start.sh

The fix to check for TACHYON_MASTER_JAVA_OPTS and TACHYON_WORKER_JAVA_OPTS need to be wrapped with double bracket "[[" and "]]" to prevent
error message in tachyon-start.sh when it is containing spaces or extra characters like:
export TACHYON_MASTER_JAVA_OPTS="$TACHYON_JAVA_OPTS -agentlib:jdwp=transport=dt_socket,server=y,suspend=n,address=6001"
Merge pull request #339 from hsaputra/escape_check_masterandworkerjavaopts

TACHYON-63 Remove script error "[: too many arguments" in tachyon-start.sh