piuparts_wrapper: quote scriptsdir in piuparts cmd

scriptsdir can be filled by the user and might ends up containing
spaces. Make sure its content is passed as one argument by double
quoting the variable when passing it to the piuparts call.
piuparts_wrapper: unset scripts dir when non existent

On Ubuntu Precise (Long Term Support release), the piuparts script does
not comes with /etc/piuparts/scripts which causes the piuparts output to
report:

 0m0.0s ERROR: Scripts directory is not a directory:
 /etc/piuparts/scripts/

Add a check to make sure it is a directory, if it is not, fallback to an
empty string passed to piuparts.