Using vendor metadata helps alleviate the need to spin custom images
for things like package mirrors, timezones, or network proxies.
Adds new config option 'vendor-data' which takes a JSON formated
string to be used as static vendor metadata.
Adds new config option 'vendor-data-url' which takes a URL which
serves dynamic JSON formatted vendor metadata.
Adds new NovaMetadataContext class which writes
/etc/nova/vendor_data.json and enables it via nova.conf.
Closes-Bug: 1777714
Change-Id: I1d70804e59d42b0651a462c81e01d9c95626f27d
b355ea0473