Fix list of modules not included in auto-gen docs

The variable for excluding modules should be a sequence of strings.
Turn the current value from a string to a tuple so documentation
for our source is generated.

The errors and warnings were fixed in the docstrings in the source code
otherwise the doc generation would fail.

The following files were excluded from the doc build because of
non-existing imports:

 'ceilometer.compute.nova_notifier'
    http://bit.ly/remove-nova-notifier-bp
 'ceilometer.openstack.common.db.sqlalchemy.session'
    https://review.openstack.org/#/c/97850/
 'ceilometer.openstack.common.middleware.audit'
 'ceilometer.openstack.common.middleware.notifier'
    https://bugs.launchpad.net/ceilometer/+bug/1327084
 'ceilometer.openstack.common.log_handler'
    https://bugs.launchpad.net/ceilometer/+bug/1327076

These failed imports are registered in one blueprint and two bug reports.

Change-Id: If0bc1c8fc96ba513bbeb90d5257e40b7621a8473
Merge "Fix list of modules not included in auto-gen docs"