cephadm: split default_image decorator into two functions
Keep default_image as a decorator for functions that will only
ever need to update an image passed by the CLI. For other future
functions that want to execute code prior to assigning an image
from CLI parameters add `update_default_image` which takes a
CephadmContext and updates it and can be used by the caller
at an arbitrary point in the code flow.
Signed-off-by: John Mulligan <jmulligan@redhat.com>