Add simple, scalar options to allow additional private IP addresses to be assigned with run-instances.

This customization adds two new parameters to the ``ec2 run-instance``
command.  The first, ``--secondary-private-ip-addresses`` allows a list
of IP addresses within the specified subnet to be associated with the
new instance.  The second, ``--secondary-ip-address-count`` allows you
to specify how many additional IP addresses you want but the actual
address will be assigned for you.

This functionality (and much more) is also available using the
``--network-interfaces`` complex argument.  This just makes two of
the most commonly used features available more easily.

This PR is dependent on https://github.com/boto/botocore/pull/106