Enhanced mount module to make 'src' optional

Enhancement of "mount" module from Seth Vidal : 
- Made 'src' parameter optional : 
  I wanted to be able to act on lines in fstab like this : 
  * SERVER 1 = /dev/mapper/vg0_lv_home /home ext4 defaults 1 2
  * SERVER 2 = /dev/mapper/LogVol00/lv-home /home ext4 defaults 1 2
where 'src' are different but 'name' is the same. If 'src' is provided, a test is made like before, elsewhere it looks only at 'name' parameter.
- Added a parameter 'delmp' (means "delete mount point", can be rename if you want) for the unset_mount and unmount functions, because removing the directory under the mount point, without ask for it before, was too dangerous, IMHO (think : for some reason I want to umount /tmp, but I don't want to remove the directory !!)
- Added 2 states only_if_present and only_if_mounted to modify a line ONLY IF it is present in fstab.

Tested here, please test and validate if you find this useful.