bash-completion: fix completing  without dnf-plugins-core (RhBug:1151231)

dnf list is printing some info which we should not parse (especially
with debug - `Cleaning` string). We want use only packages from output.
Let's find string `Available Packages` and print all packages before
next info string `Cleaning`. If user will have some custom plugins which
will modify `list` command - completion will be break, but we can
guarantee that with `dnf` and with/without dnf-plugins-core it will work
OK.

Reference: https://bugzilla.redhat.com/show_bug.cgi?id=1151231
Signed-off-by: Igor Gnatenko <i.gnatenko.brain@gmail.com>