Added absolute paths to all commands

Changed all commands used by the script to call other applications/utilities so that only absolute paths are used. This was done to make it possible to use the script as a cron job as is.

The following paths are now used:

mail   : /bin/mail
vzlist : /usr/sbin/vzlist
vzctl  : /usr/sbin/vzctl
rsync  : /usr/bin/rsync
nice   : /bin/nice
ionice : /usr/bin/ionice

These paths have only been tested on CentOS 6.5. They may be different
in other distributions.