rename port attribute variable to SECURITYGROUPS from SECURITYGROUP

This patch renames the port attribute variable to the plural form since
multiple security groups can be associated with a port. This patch does
not change the security groups api and also does not change the
configuration section name to be SECURITYGROUPS.
Fixes bug 1095885

Change-Id: I1a4df337b48da0ef9c94d74b04c0b86333ce2f0c
Merge "rename port attribute variable to SECURITYGROUPS from SECURITYGROUP"