Parse JSON in ovs_lib.get_vif_port_by_id

This patch replaces regex matching of text output with parsing
of JSON output in ovs_lib.get_vif_port_by_id.
This makes the code more reliable as subtle, possibly even
cosmetic, changes in ovs-vsctl output format could cause the
regular expression match to fail.

Also, this makes the code consistent with ovs_lib.get_vif_port_set
which already uses JSON output.

Finally this patch slightly changes the behaviour of
ovs_lib.get_vif_port_by_id returning None if elements such as
mac address or ofport were not available.

Change-Id: Ia985a130739c72b5b88414a79b2c6083ca6a0a00
Closes-Bug: #1280827
Merge "Parse JSON in ovs_lib.get_vif_port_by_id"