unattended_install: Fix installation for user networking

With commit bd50d1d9, we changed the behavior of the
API utils_net.get_ip_address_by_interface, so that
it throws an exception if we could not get the interface.

This broke unattended_install when using userspace nw,
so let's fix it by handling the exception and assigning
None to the auto_ip in case the call fail, reproducing
the original (pre bd50d1d9) behavior.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
run: Reorganizing option parsing, clarifying flags usage

We did miss the fact that a bunch of our new command
line flags were breaking -c usage, given that those
new flags parsed "only" filters to the existing cartesian
configurations.

So reorganize the general options parsing, and carefully
review which options can go with -c and which ones will
be ignored, documenting this on --help output.

This also resolves one of the requests from Igor Mammedov,
of not compulsorily setting --qemu-bin when using custom
config files. So if -c is provided and no --qemu-bin was
provided in the command line, no attempt to set it will
be made. Of course, that was also explained in the --help
output.

CC: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>