network: Use "prefix length" for IPv6

Previously the term "Netmask" was used incorrectly for the IPv6
prefix length. The name is now corrected and additionally only an
integer is allowed as the prefix length.

Change-Id: I18e7829e7c9e4b265822d02aa86c34f8fda41519
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=981168
Signed-off-by: Fabian Deutsch <fabiand@fedoraproject.org>