partly revert e3bb2219: properly save drush output

the commit's assumption was incorrect: HOSTING_DRUSH_OUTPUT is the
actual return value of a single drush_backend_invoke() call, not a
list of such output or the actual drush output (that's the 'output'
field of the return value).

so we need to set HOSTING_DRUSH_OUTPUT to the output of the actual
task, and that's it. we can get the log output of the provision-save
commands by using the integrate parameter, which this patch does.

see #2471108