cephadm: add fetch_meta function for reading metadata properties from ctx

cephadm: add fetch_meta function for reading metadata properties from ctx

The CephadmContext holds much of cephadm's state, including values from
cli options and (soon) configuration file input. Add a `fetch_meta`
function that reads processed metadata from `meta_properties` or parses
JSON stored on `meta_json`. Parsed JSON is saved to `meta_properties`.
Future code should always use fetch_meta for reading, and only ever
set `meta_properties` as a python dict, not round trip JSON through
meta_json.

Signed-off-by: John Mulligan <jmulligan@redhat.com>