ENH: DRM to DRMAA configuration adapters

[jerdra]

This PR implements configuration adapters so that we can specify scheduler configuration options directly in the Airflow Operator definition using a wrapper configuration object, i.e:

# Specify using DRMAA job configuration
AirflowOperator(
   ...,
   executor_config=DRMAAConfig(**DRMAA_OPTIONS)
)

# Specify using SLURM job configuration
AirflowOperator(
   ...,
   executor_config=SlurmConfig(**SLURM_OPTIONS)
)

This can be easily extended to accommodate any HPC scheduler with a DRMAA interface written for it