DirectoryUpload causes master to disconnect from Crossbar

Large files transferred via DirectoryUpload step can block the master and finally causing it to disconnect from Crossbar.

To reduce the load on master, instead of:
https://github.com/MariaDB/buildbot/blob/d5136c7dce664a65ae8932712da3298af34cf3d7/master-docker-nonstandard-2/master.cfg#L338
where DirectoryUpload will transfer the data to the master and then the master will write it to ci.mariadb.org

we should rather switch to the standard approach of using a network attached storage, implemented by saveLogs() function in:
https://github.com/MariaDB/buildbot/blob/d5136c7dce664a65ae8932712da3298af34cf3d7/master-docker-nonstandard-2/master.cfg#L670

this will keep the load on the worker and the master will only be responsible of retrieving the stdout/err logs.