mgr/cephadm: add exchange types and use them for deploy

mgr/cephadm: add exchange types and use them for deploy

Add an exchange module for defining the "exchange protocol" that the
mgr module uses for communicating with the cephadm binary.  The
exchange module classes use data descriptors to define supported
fields in the resulting data and will be serialized to JSON for
communicating with cephadm.

The cephadm binary does not use these types because it currently
doesn't import anything outside of the standard library. A suggested
future change would be to move 'exhange.py' to python-common somewhere
and have the module be imported by both the mgr and cephadm and used
for both serialization and deserialization.

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