Enforcement of the symlink creation if the 'force' parameter is set to 'yes'

Dear Ansible Dev Team,

This pull request brings the enforcement of symlink creation in Ansible if the parameter named 'force' is set to 'yes' in two cases:
- the source file does not exist (but will appear later).  
Example: `action: file state=link path=/var/www/html/animation.gif src=/dev/shm/animation.gif force=yes`
  where /dev/shm/animation.gif is a file, which should be generated later by the script.
- the destination exists and a file (so, we need to unlink the "path" file and create symlink to the "src" file in place of it):  
Example: `action: file state=link path=/etc/localtime src=/usr/share/zoneinfo/Europe/Zurich force=yes`