libglusterfs: make gf_string2percent_or_bytesize to convert float numbers.

Change-Id: I0ac6588297bdb8b085c3c9b01848a098f7fd4293
BUG: 912206
Signed-off-by: Varun Shastry <vshastry@redhat.com>
Reviewed-on: http://review.gluster.org/4533
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Anand Avati <avati@redhat.com>