Including tasks has a performance penalty when compared with importing
tasks. The nova-cell role uses include_tasks twice when generating
certificates and keys for libvirt TLS. While a dynamic include makes
sense here for a non-default feature, we can use one include rather than
two with the same effect. Since this task runs against compute nodes the
overhead is significant.

See [1] for benchmarks of include_tasks and import_tasks.

[1] https://github.com/stackhpc/ansible-scaling/blob/master/doc/include-and-import.md

Partially-Implements: blueprint performance-improvements

Change-Id: Ic687d2f7d4625aede386e576ebb174da72142756