I was a bit surprise this command didn't simply fetch the files, specially taking care SRC_URI is always the same (I mean, it doesn't change depending on the USE flags for example):
$ emerge -Of nova
!!! Problem resolving dependencies for sys-cluster/nova
!!! The ebuild selected to satisfy "nova" has unmet requirements.
- sys-cluster/nova-13.1.1::gentoo USE="compute memcached novncproxy rabbitmq -compute-only -iscsi -mysql -openvswitch -postgres -sqlite" ABI_X86="64" PYTHON_TARGETS="python2_7"
The following REQUIRED_USE flag constraints are unsatisfied:
!compute-only? ( any-of ( mysql postgres sqlite ) )
The above constraints are a subset of the following complete expression:
!compute-only? ( any-of ( mysql postgres sqlite ) ) compute-only? ( compute !rabbitmq !memcached !mysql !postgres !sqlite ) any-of ( python_targets_python2_7 )