ui: add URL for static configurations to load a specified file

In certain configurations, e.g. automation, we want to use static
networking but load a particular file, automatically and parse it as a
pxelinux config file. Currently, we support something like this for DHCP
based booting, but not static. Add a URL field to the UI for static
configurations and reuse the logic from device_handler_process_url() to
load the specified file.

Signed-off-by: Nishanth Aravamudan <nacc@linux.vnet.ibm.com>
Signed-off-by: Samuel Mendoza-Jonas <sam.mj@au1.ibm.com>