(maint) log uid and gid for Puppet::Util::Execution.execute if specified as non-default

This might be able to use a little bit of cleanup, I guess it depends on how nicely you want the output formatted. But I was debugging a custom provider that uses Puppet::Util::Execution.execute and seemed to be executing as root instead of the specified user... and found that the debugging output didn't really help me find the problem.
@peterhuene Sure, do you have any suggestions for the formatting? Would this be better (building the string for all 4 possible conditions is a bit of a pain):

Executing:
Executing with uid=X:
Executing with gid=Y:
Executing with uid=X gid=Y: