supervising/supervisor:

Don't check for failed nodes when OQ_NO_DISTRIBUTE is enabled. Otherwise,
this can cause spurious errors and failures in the supervisor.
supervising/supervisor:

Added a comment explaining why the failed node checked is skipped in certain
cases.
Fixed a test calling an incorrect function
Merge pull request #1006 from gem/1099801

Fix PickleFieldTestCase.test_get_prep_value
Merge pull request #1007 from larsbutler/superv-support-for-no-distribute

Supervisor support for `no_distribute`

[r=matley]
Merge branch 'master' of https://github.com/gem/oq-engine into site_model_validation_bug_fix
removed old comment and unused import