phase1: add config option for shared workdir

phase1: add config option for shared workdir

This patch adds a per-slave configuration option:
    shared_wd = <boolean>

This option defaults to False if unset, and should be set to True for
slaves that share a single workdir among all workers.

When it is True, expires.sh and cleanup.sh are disabled as the workdir
housekeeping is entirely handled by the Git() step and the value of the
'do_cleanup' configuration option.

expires.sh and cleanup.sh are unnecessary and potentially detrimental in
this context.

Signed-off-by: Thibaut VARÈNE <hacks@slashdirt.org>