add doc/source/api in gitignore
add doc/source/api in gitignore

Some files are created after running 'python setup.py build_sphinx'.
These should be ignored by git

Change-Id: I89d671d84294169dac5aacf18800ab805605aa9f

sigmavirus24/glance,openstack/glance,ntt-sic/glance,saeki-masaki/glance,redhat-openstack/glance,ozamiatin/glance,darren-wang/gl,scripnichenko/glance,dims/glance,jumpstarter-io/glance,cloudbau/glance,JioCloud/glance,tanglei528/glance,takeshineshiro/glance,SUSE-Cloud/glance,rickerc/glance_audit,JioCloud/glance,klmitch/glance,wkoathp/glance,rickerc/glance_audit,redhat-openstack/glance,dims/glance,akash1808/glance,cloudbau/glance,vuntz/glance,openstack/glance,akash1808/glance,tanglei528/glance,rajalokan/glance,sigmavirus24/glance,stevelle/glance,klmitch/glance,stevelle/glance,paramite/glance,citrix-openstack-build/glance,citrix-openstack-build/glance,ntt-sic/glance,rajalokan/glance,jumpstarter-io/glance,wkoathp/glance,paramite/glance,SUSE-Cloud/glance,takeshineshiro/glance,saeki-masaki/glance,vuntz/glance,kfwang/Glance-OVA-OVF,ozamiatin/glance,kfwang/Glance-OVA-OVF,openstack/glance,scripnichenko/glance,darren-wang/gl