Make the ``host`` resource attribute on the ``instance`` resource type
optional in Gnocchi.
This allows the hypervisor a compute instance is running on to be
withheld from Gnocchi's resource metadata, which may be required
for security reasons e.g. for public clouds.
To handle updating existing attributes, changes have been made to
the ``upgrade_resource_types`` function to make it check if any
existing attributes are identical to the specified schema, and
update the resource type accordingly.
Change-Id: I26875b876be6401164442d2f5e52e2eb9b8e703c
703ada2c05