Allow mount source to be relative to task working directory

[alemonmk]

Per hashicorp/nomad#13229 (comment) and #123 we can mount stuff from ${NOMAD_TASK_DIR}/file with relative path local/file but not for ${NOMAD_TASK_DIR}/secrets. I think it should be allowed to have a consistent behavior, and be of convenience to some container that is not easy to change their config seeking behavior.

For reference here's how docker driver doing this for volume bind and simple bind:
https://github.com/hashicorp/nomad/blob/2697e63ad67c254d0d8f1be02a477807fe40c50a/drivers/docker/driver.go#L678-L689
https://github.com/hashicorp/nomad/blob/2697e63ad67c254d0d8f1be02a477807fe40c50a/drivers/docker/driver.go#L1253-L1263