Add security_groups for create_port()

Add security_groups for create_port()

Currently if defined SecurityGroups and SubnetId properties for
instance in heat template, the SecurityGroups were not associated
to the port created within the previous SubnetId, in another word,
the instance are not allocated to the specified security groups
defined in heat template.

    * Modifies function _build_nics() in instance.py, adding an item
'security_groups' in the post body of create_port sending by neutronclient.
    * Adds a new function _get_security_groups_id() to map security_groups
names to ids.
    * Adds corresponding unit tests.

Fixes bug #1221564

Change-Id: Ica2df7c6f96bc982a5bd5ece3611949ae905e5c8