bin/openquake:

bin/openquake now sets the OQ_NO_DISTRIBUTE envvar when the
`--no-distribute` cmd line switch is specified.
__init__:

Added a util function for checking the state of the OQ_NO_DISTRIBUTE envvar.
calcs/base:

`execute` now logs a WARNING when calculation distribution is disabled.
calcs/base:

`queue_next` now executes tasks as plain functions if OQ_NO_DISTRIBUTE is set.
engine2:

`_switch_to_job_phase` now checks if OQ_NO_DISTRIBUTE is set. If it is,
don't check the presence of worker nodes. (In the default case, an absence
of workers will cause the calculation to abort immediately. In the
non-distributed case, this check is skipped.)
calcs/hazard/classical/core:

Mean/quantile post-processing code now takes into account the "no-distribute"
option.
calcs/hazard/classical/post_processing:

Hazard map post processing now takes into account the "no-distribute"
option.
qa_tests/_utils:

Hazard QA tests turn off distribution by default.
qa_tests/_utils:

Cleaner patching for `OQ_NO_DISTRIBUTE` envvar.
tests/openquake_test:

Added some tests for `no_distribute()`.
qa_tests/_utils:

Removed unused import.
Merge pull request #1002 from larsbutler/distribution-switch-2

Distribution switch, take 2

[r=micheles] [f=1097862]