Some resources do not work if their metadata is in
a wrong state, .e.g the metadata 'scaling_in_progress'
of scaling group/policy might be always True if engine
restarts while scaling.
This patch adds an interface 'handle_metadata_reset' for
resource, then the plugins can override it if needed.
We reset the metadata while marking resource healthy.
Change-Id: Ibd6c18acf6f3f24cf9bf16a524127850968062bc
Closes-Bug: #1651084