packaging: Application Mode option in installer

Application Mode option added to engine-setup, Engine can be
configured as gluster only, virtualization only or both.

This new prompt can be disabled by setting
USE_DEFAULT_APPLICATION_MODE_WITHOUT_PROMPT to True in basedef.py
Also introduced a Constant(CONST_DEFAULT_APPLICATION_MODE)
in basedef.py to set the default application mode.

Now USE_DEFAULT option can be dynamic, so function name can be
specified instead of value. Initially USE_DEFAULT contains
True or False, now function name can be given instead of
Boolean which helps to decide usage depending on the other
questions answered.

Modified NFS options override in plugins/all_in_one_100.py since
those options can be managed by setting USE_DEFAULT dynamically.

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=901443

Change-Id: I2c958205727905d00f6e407779ee64f5c5eb5a1a
Signed-off-by: Aravinda VK <avishwan@redhat.com>
(cherry picked from commit 3dde9fbe2792e54d5c6c93d0e88cf8ab84f95367)