Expose AbortCurrentUpgrade over the API

This will be used by the user to clear the synchronisation document
for a previously failed upgrade (via an arg to "juju upgrade-juju").
drive-by: moved serverSuite

serverSuite and it's methods was sitting in the middle of
clientSuite. This was a tad confusing.
Merge pull request #738 from mjs/AbortCurrentUpgrade-api

Expose AbortCurrentUpgrade on API

This will be used by the user to clear the synchronisation document for a previously failed upgrade in the rare case of a partially completed upgrade.