Handle constraint equality for bundle dry run

[howbazaar]

The comparison of contraints was incorrect in the bundle deployment.

The struct equality was not sufficient, so we now use the string representation of the parsed constraints.

QA steps

juju deploy hadoop-processing
# Wait until everything is stable. 
juju deploy hadoop-processing --dry-run

There should be no steps to update constraints.

[howbazaar]

$$merge$$

[jujubot]

Status: merge request accepted. Url: http://ci.jujucharms.com/job/github-merge-juju