Make build_userdata user_data_format aware

Over time different user_data_format values could assemble
different combinations of cloud-init parts.

As the default will remain user_data_format='HEAT_CFNTOOLS',
identical content will be generated for the Instance resource.

The Server resource now passes user_data_format, and no longer
has a get_mime_string method since build_userdata is only ever
called once so cached get_mime_string results is not needed.
Change-Id: I7cfbcffeea24c9b1f013463f9a5fa53b8c699cfe
Instance call build_userdata directly

get_mime_string is only ever called once so caching the results
in self.mime_string is not necessary.

Change-Id: I36a1eb358b1228a946c5f8fa6e76869f10c398fd
Merge "Make build_userdata user_data_format aware"
Merge "Instance call build_userdata directly"