Default Bosh::Dev::Build.candidate bucket name

As of 3f5227b, the bucket name would be required by the 9 other callers of
Build.candidate. Instead, we default it to bosh-ci-pipeline, which is what it
used to be, and move the logger (which was specified in only one call of
Build.candidate) out of the arguments and into the method.

Signed-off-by: Edward Zhang <zhuadl@cn.ibm.com>