cephadm: add fetch_configs function for reading configuration items from ctx

cephadm: add fetch_configs function for reading configuration items from ctx

The CephadmContext holds much of cephadm's state, including values
from cli options and (soon) configuration file input. Add a
`fetch_configs` function that reads processed configuration items from `config_blobs`
(a dict mapping config name to arbitrary configuration content) or
falls back to using the `get_parm` function on `ctx.config_json`.

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