make get_port_mask more efficient by avoiding some unnecessarily mask checking

[ding bo]

get_port_mask function is used to aggregate port range into masks to install fewer flows in ovs. But, this function literates all possible masks to check if it is needed. In fact, some iterate can be avoided to make this function better time complexity.

original code here:
https://github.com/openstack/networking-sfc/blob/master/networking_sfc/services/sfc/common/ovs_ext_lib.py#L35

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/338741

[OpenStack Infra (hudson-openstack)]

Change abandoned by Armando Migliaccio (<email address hidden>) on branch: master
Review: https://review.openstack.org/338741
Reason: This review is > 4 weeks without comment, and failed Jenkins the last time it was checked. We are abandoning this for now. Feel free to reactivate the review by pressing the restore button and leaving a 'recheck' comment to get fresh test results.