cephadm: split command_deploy_from into mockable parts

cephadm: split command_deploy_from into mockable parts

Split up command_deploy_from so that the part that reads the JSON
from an input source is moved into read_configuration_source and
the part that applied the configuration data to the ctx is moved
into apply_deploy_config_to_ctx. This will make these sections
reusable and, more importantly, make it easier to mock/monkeypatch
these functions in tests.

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