Don't try to rm nothingness

Currently if we don't use random mockroot, we try to rm an uninitialized
variable, which breaks in funny ways depending on your ruby version. Here we
explicitly set confdir to nil and then rm it later only it isn't nil.

Signed-off-by: Moses Mendoza <moses@puppetlabs.com>
Merge pull request #121 from MosesMendoza/maint/master/fix_rm_configdir

Don't try to rm nothingness