README: Mention the need for setting the path appropriately

Without at least /sbin, /bin, /usr/sbin and /usr/bin, the
setmysqlpass.sh script won't run correctly and the module will not set
the requested root password.
setmysqlpass: be more careful before plundering into action

Since this script is rooting the database, it'd be good to use a little
more precaution so that we don't let systems be in an inconsistent case
when crashing.

In cases where the PATH variable is not appropriately set (variable is
empty by default when script is invoked by puppet) the script shuts down
mysql and then is not able to call most commands.
manage configuration snippets in a conf.d directory

this should make it easier to override values from my.cnf, or to set
other values that aren't in the main config file.
conf: notify mysql service to make changes effective
Document the new define in the README

Signed-off-by: Gabriel Filion <gabster@lelutin.ca>
Merge branch 'conf_define'
Merge branch 'mention_path'