webadmin: Move SizeConverter to more general location

Class SizeConverter is too general to be located under gluster package.
New location make it easier for reuse.

Change-Id: I684904feeb4e0ea1b0844a82629cd5b36ff61cb8
Signed-off-by: Sergey Gotliv <sgotliv@redhat.com>
webadmin: Using SizeConverter.SizeUnit by DiskSizeRenderer

1.Removes DiskSizeRender.DiskSizeUnit enum and start using
SizeConverter.SizeUnit instead. Both enums are representing disk size
units but SizeUnit is used by SizeConverter utility when converting the
size from one unit to another.

2.Signature of SizeConverter convert function is changed to return
general Number and not only long as it was before. It allows to return
numbers with floating point.

Change-Id: I1d1820aaa246577f8d39a6826c04b31771c1ac06
Signed-off-by: Sergey Gotliv <sgotliv@redhat.com>