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.

Change-Id: Ieba515d2baf3559bf27185c7c6432160551ef172
Signed-off-by: Aravinda VK <avishwan@redhat.com>