cephadm: add a pythonic build.py for constructing the cephadm binary

cephadm: add a pythonic build.py for constructing the cephadm binary

As discussed in a ceph orch meeting, the build.sh script was deemed
"unpythonic". This script is a first attempt to do it more pythonic
with fewer explicit version checks.

This change minimizes the existing build.sh to simply call build.py.
We can completely eliminate build.sh at a later time if desired.

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