restapi: Validate boolean values

JAXB doesn't validate boolean values, anything that isn't the string
"true" is interpreted as false. This patch introduces a custom parser
for the boolean type that only accepts the strings "true" and "false",
any other thing will be rejected.

Change-Id: Ibb3558185a46869a73115b936183cb22007c99e9
Bug-Url: https://bugzilla.redhat.com/1063876
Signed-off-by: Juan Hernandez <juan.hernandez@redhat.com>