This patch adds the ability to store and retrieve the
ProvisionedThroughput in a table.
The information is stored in the table tags. We use the TTL convention
used in alternator, and the tags will be: system:provisioned_rcu and
system:provisioned_wcu.
verify_billing_mode function now return a struct with the billing mode
information.
The code of describe_table now check if the provision tags exists and
return the RCU and WCU accordingly.
Signed-off-by: Amnon Heiman <amnon@scylladb.com>