Enable specific extra_dhcp_opt to be left blank

Enable specific extra_dhcp_opt to be left blank

In some cases there is a need for a blank value of the "router" extra
dhcp option. This fix addresses this issue by introducing a simple
mechanism in the code that allows specific extra dhcp options to be left
blank by applying a different opt_value validation of the data received
from the REST call and different validation on the value that will be
written to the DB.

This fix also takes into consideration bug #1257467 which claims that
in case of a blank "server-ip-address" option a segmentation fault
occurs in dnsmasq. I did not check this claim with a newer dnsmasq
version since it seemed logical that the list of potentially blank
options should be limited to options that are known to work well when
blank and have functional justification for being blank (e.g. "router")

APIImpact
"router" and "classless-static-route" extra dhcp options can be blank
(e.g. opt-name="router", opt-value="")

DocImpact
During port creation/update, Specific extra-dhcp-options can be left
blank ("router" and/or "classless-static-route"). This causes dnsmasq
to have an empty option in the "opts" file related to the network to
which the port is related.
For example:
tag:tag0,option:classless-static-route,
tag:tag0,option:router,

Closes-Bug: #1437695
Related-Bug: #1257467

Change-Id: I0ac76e132c4bd86da39863674d4cf8a22dad7034