Allow plugins to express dependency info

Add a no-op function, "plugin_requires" to allow plugins to indicate
their dependencies on each other.  This will be used by the Devstack
Ansible module when writing local.conf files.

Also add define_plugin to allow plugins to indicate their canonical
names.

Change-Id: Ibd8c7222ed7dfb08d7ea821d871fc6f3b88de24b
2 files changed