You will need to create the SDdraft files and then modify them using the minidom library before you publish. There is an example of this in the 10.3 help. See 'Modify SDDraft example 1': http://desktop.arcgis.com/en/arcmap/10.3/analyze/arcpy-mapping/createmapsddraft.htm

From ESRI's example, a service's description is changed to 'US Counties Map':

<!-- language: lang-py -->

    import arcpy
    import xml.dom.minidom as DOM 
    # the new description
    newDesc = 'US Counties Map'
    xml = r"C:\Project\Counties.sddraft"
    doc = DOM.parse(xml)
    # find the Item Information Description element 
    descriptions = doc.getElementsByTagName('Description')
    for desc in descriptions:
    if desc.parentNode.tagName == 'ItemInfo':
        # modify the Description
        if desc.hasChildNodes():
            desc.firstChild.data = newDesc
        else:
            txt = doc.createTextNode(newDesc)
            desc.appendChild(txt)
    # output to a new sddraft
    outXml = r"C:\Project\Output\CountiesForWeb.sddraft"
    f = open(outXml, 'w')
    doc.writexml( f )
    f.close()