Avoid type error in crush rule (str, not an int)

The crush rule is passed to create_pool as a str, but later converted to a c_ubyte when passed to the underlying C implementation. c_ubyte does not convert directly from str, so we must parse it as an int first.

Previously, create_pool would throw "an integer is required" exception when called with a crush rule.

Signed-off-by: Jeff Epstein <jeff.epstein@commerceguys.com>