Fix the replacement placeholder in string

Bug #1096721

Change-Id: Ief6f410e4532772af3f9775b48ce92ac901f812d
change xxx_metadata_agent() into xxx_metadata_proxy()

Bug #1092047

Change-Id: Ic59da98105a7301697ecb5a6477f3c7fc3f8e86a
Add comment to indicate bridge names' length

Bug #1096652

Change-Id: Ic814b59da131fba488b728eb3b67864712cde141
Merge "Add comment to indicate bridge names' length"
Merge "Fix the replacement placeholder in string"
Enable the user to enforce validity of the gateway IP

Fixes bug 1096532

A new configuration variable is added to enable the user to indicate
if the gateway should be validated on the subnet. For backward
compatibility this is set as False by default.

Change-Id: Ieadd60a945d34703bfee7576aa3b2ff7da3143d4