add a default value for the -d option of ansible-pull

Thi permit to simplify the command line to use by using
a sensible default, and so reduce the number of incorrect
possible choices and setup needed. Among potential
incorrect choices is using a fixed directory in /tmp, which
could be problematic with a setup whose access is not
properly restricted.
Merge pull request #4576 from mscherer/pull_temporary_dir_2

add a default value for the -d option of ansible-pull