Tweaked $::osfamily calls in init.pp and params to make the module work on SLES 11

osfamily fact has no value on SLES 11 SP3 during my tests,
consequently SLES is not recognized as belonging to
the Suse osfamily. Then I added a test in params.pp to give a
value to $snmp::params::osfamily which is 'Suse' if $::operatingsystem
is 'SLES' or $::osfamily if not. I also changed calls for $::osfamily
to $snmp::params::osfamily. Maybe this problem is due to an inconsistent
reason or because my tests are made on virtual machines (??). For now
this does the trick for me.