Script janusgraph.sh fails for 0.6.x when not called from the project root

The script janusgraph.sh fails for 0.6.x when it is not called from the project root.

- Version: 0.6.x
- Storage Backend: cassandra
- Mixed Index Backend: elasticsearch
- Link to discussed bug: https://lists.lfaidata.foundation/g/janusgraph-users/topic/89549434
- Expected Behavior: script can be called from any location
- Current Behavior: Script janusgraph.sh fails for 0.6.x when not called from the project root
- Steps to Reproduce: $ janusgraph-full-0.6.1/bin/janusgraph.sh start

### Stack Trace (if you have one)
Follow link above to user list.

Start with janusgraph-full-0.6.1/bin/janusgraph.sh and reproduce the failure by running `janusgraph.sh start` from outside the project root. The work is done when the script starts successfully from any location, including the reported 0.6.x setup.

Written by the indexing model from the issue text.