Make /usr/bin first in path to ensure that system python is used

On Ubuntu, /usr/local/bin is listed before /usr/bin based on /etc/environment:
$ cat /etc/environment 
PATH="/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games"

On some systems, a different version of python could be installed outside of the package manager to /usr/local/bin/python.  The different version would not contain the salt system package, causing /usr/bin/salt-minion to fail to import salt when the service is started.

Although the real issue is the non-system python installation, the salt-minion service should always invoke system python.