slap/interface/slap: pep8, typos

checked with:
  pylint --disable all --enable spelling --spelling-ignore-words=URI,SSL,API,XXX,uninstall,initialize,slapgrid,slapformat,kvm,instantiation,IComputer,IException,Vifib,IRequester,ISupply,SocketError,virtio,boolean,traceback,http,mysoftwareproduct,foo,buildout --spelling-dict en_US slapos/slap/interface/slap.py

/reviewed-on nexedi/slapos.core!66