Fix missing specifying doctrees directory
Fix missing specifying doctrees directory

In tox.ini file, the doctrees directory is specified
as an argument of sphinx-build command in api-guide, api-ref,
placement-api-ref, releasenotes targets.
But it is missing in docs. So add it.

TrivialFix
Change-Id: I74ea98b1330c1c550dae53fafc79bd6e41727296

gooddata/openstack-nova,mahak/nova,mikalstill/nova,mahak/nova,mikalstill/nova,klmitch/nova,gooddata/openstack-nova,openstack/nova,klmitch/nova,rahulunair/nova,gooddata/openstack-nova,klmitch/nova,openstack/nova,openstack/nova,gooddata/openstack-nova,rahulunair/nova,rahulunair/nova,klmitch/nova,mahak/nova,mikalstill/nova