importer config needs to be more DRY

[Steve Langasek]

We currently have importer config lines on system-image that look like this:

file_custom,krillin = http;http://s-jenkins.ubuntu-ci:8080/view/cambridge/job/BQ International Tarball - Gated/lastSuccessfulBuild/artifact/custom.tar.xz;name=custom,monitor=http://s-jenkins.ubuntu-ci:8080/view/cambridge/job/BQ International Tarball - Gated/lastSuccessfulBuild/artifact/build_id

This is completely unreadable and typo-prone.

It would be better if the 'monitor' option could take a relative path, as an alternative to an absolute URL. E.g.:

file_custom,krillin = http;http://s-jenkins.ubuntu-ci:8080/view/cambridge/job/BQ International Tarball - Gated/lastSuccessfulBuild/artifact/custom.tar.xz;name=custom,monitor=build_id

This should be interpreted as a path relative to the associated URL:
 - if the 'monitor' value doesn't begin with ([a-zA-Z]*://|/)
  - strip off the part of the URL after the last '/' and append the monitor value to make the full monitor URL