script/ceph-backport.sh: wholesale refactor

script/ceph-backport.sh: wholesale refactor

This commit refactors the script to make it more user-friendly and
maintainable. Added features:

* script now determine release/milestone from backport issue instead of
  requiring user to provide it
* script now generates a more verbose PR description
* backport PR title is based on original PR instead of the tracker issue
* improved error handling
* new --debug option that triggers "set -x"
* new --verbose option
* errors and diagnostic messages are now printed to stderr

Signed-off-by: Nathan Cutler <ncutler@suse.com>