SPANK env var name change

SPANK naming changes: For environment variables set using the
    spank_job_control_setenv() function, the values were available in the
    slurm_spank_job_prolog() and slurm_spank_job_epilog() functions using
    getenv where the name was given a prefix of "SPANK_". That prefix has
    been removed for consistency with the environment variables available in
    the Prolog and Epilog scripts.
bug 1570