Validate sizes on storage installer page and calculate free space

Make sure users cannot proceed with values which are too large.
Additionally, dynamically update either the size of the data
partition or remaining free space, depending on whether a
checkbox to fill remaining space with the data partition is
selected or not

Change-Id: Ibde087a31909d26aef8e00b58d895567e87b12cd
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=918961
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=867843
Signed-off-by: Ryan Barry <rbarry@redhat.com>