Fix 'timeout' parameter of glance-image module.

The timeout parameter of glance-image was not being parsed into a
numeric type, causing the following error when specifying timeout:

    msg: Error in creating image: unsupported operand type(s) for +: 'float' and 'str'
Merge pull request #3648 from echohead/glance-image-timeout

Fix 'timeout' parameter of glance-image module.