Add support for float values in update_value() helper

This is needed to add support editing scheduler attributes
(cpu_allocation_ratio/ram_allocation_ratio) in the nova barclamp.
See e.g.:
https://github.com/SUSE-Cloud/barclamp-nova/commit/03623e9fbbd656509480c1a36e322c5d8ffce595

Signed-off-by: Vincent Untz <vuntz@suse.com>
(cherry picked from commit ca51b3996029ac96a6258a256cc373bfaffb8d6e)