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