network_page: Add hostname validation length

The commands hostname or hostnamectl accepts up to 64 chars.
This patch adds a validation for this entry in the TUI.

Change-Id: I6c542f7ec0b68fe8531c37791362daa7580cd06c
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1077996
Signed-off-by: Douglas Schilling Landgraf <dougsland@redhat.com>