use bash -l command

- when running xsub commands, invoke 'bash -l' to load bashrc file.
  - This is necessary when PATH is set by .bashrc.
  - Users can not set environment variables when the command is invoked in a non-interactive shell.
  - Although 'bash -i' command loads bashrc, it prints an error 'can't access tty; job control turned off'.
    - To avoid this message, we used 'bash -l' instead of 'bash -i'
- Since .bashrc may sometimes print some message to stdout, we need to extract the output printed by xsub commands.
  - To distinguish the output, we insert "echo XSUB_BEGIN" just before calling 'xsub'.
  - By introducing 'XSUB_BEGIN', we can find the output created by the xsub commands.
description about xsub