With the new Fstab class from prior pull request there is an
opportunity to handle all fstab related actions to be done
by that class. This commit extends the Fstab class with an
add_entry method such that we can avoid the extra lists
holding raw fstab lines in e.g the disk builder. In the end
all fstab related data is stored in an instance of the Fstab
class. This also extends the KIWI api by an fstab management
class. Related to #1329 and #1349
3b7f5be918