(NETDEV-20) Change snmp receiver namevar to host, port, username

Without this patch the snmp_notification_receiver specifies too many
attributes for the namevar.  Technically speaking, only the hostname,
port, and SNMP username or community uniquely identify a resource on the
target device.  This is a problem because the provider logic becomes way
too complicated with so many namevar components.

This patch addresses the problem by specifying the name, port, and
username attributes as parameters.  Namevar components cannot be
properties.

This patch also removes the community attribute.  The username property
now represents both the snmp username or the snmp community depending on
the snmp version of the notification receiver.