When providing paths to plugins, the path needs to be in the scope of
the plugins container, rather than that of the host.
Here we enable that by providing the mount point through the plugin
registration and then use it when constructing request target paths.
This commit introduces support for staging volumes when a plugin
implements the STAGE_UNSTAGE_VOLUME capability.
See the following for further reference material:
4731db0e0b/spec.md (nodestagevolume)
This commit adds helpers that create and validate the staging directory
for a given volume. It is currently missing usage options as the
interfaces are not yet in place for those.
The staging directory is only required when a volume has the
STAGE_UNSTAGE Volume capability and has to live within the plugin root
as the plugin needs to be able to create mounts inside it from within
the container.
Danielle Lancashire