mgr/cephadm: make remote_executables test able to handle ast.BinOp

mgr/cephadm: make remote_executables test able to handle ast.BinOp

Was adding a line to serve.py that did

if ((datetime_now() - t).total_seconds() < 60)

and this was causing the remote_executables test to fail with

ValueError: _names: unexpected type: <ast.BinOp object at 0x7f0985c8d670>

where it seems the (datetime_now() - t) was resolving to an
ast.BinOp node which had no case in _names.

This patch makes it so the remote_executables test can also
handle these BinOp nodes and the binary operations that
could be within the node

Signed-off-by: Adam King <adking@redhat.com>