Improve 'zone add' - Support default <input> and <output> values for DNS adapters

As suggested by Havard we could improve the 'ods-ksmutil zone add' command by detecting the input type specified and in the case of a 'DNS' adapter using a default path of

/var/opendnssec/addds.xml
for the adapter file.
This will greatly reduce the command line that the user is required to type because at the moment if the user wants to use a DNS adapter they must fully specify type and input (as otherwise the input defaults to a zone file path):
ods-ksmutil zone add \
--zone <zonename> \
--in-type DNS \
--input /usr/pkg/etc/opendnssec/addns.xml \
--out-type DNS \
--output /usr/pkg/etc/opendnssec/addns.xml

can become:
ods-ksmutil zone add --zone <zonename> --in-type DNS --out-type DNS