add cores constraint & alias cpu-cores

[natefinch]

This fixes-1620056 constraints should support cores=X
Added the concept of aliases of constraint names.
Changed default name of CpuCores to cores.
Added alias from cpu-cores to cores.

This change adds a warning message that is displayed if you specify "cpu-cores" on the CLI, because any error messages will reference "cores" (and everything is displayed as cores). Unfortunately, the way our flag parsing works, there's no way to display this message if the flag parsing fails, so I have to take the constraints as a string flag and then parse during run (at which time we have the ctx and thus can output the correct message).

Most of the changes are trivial conversion of "cpu-cores" to "cores" in tests... for many places it wasn't strictly necessary, we could just use the alias, but this way if we eventually remove the alias, we won't have to update those. Plus, some of them were testing the output, which is always "cores" and it was easier to just change every instance than try to pick out the ones that mattered.

Note, this changes the output of machine hardware characteristics as well, so that change could break some people. It does not change the API representation of cpu-cores for hardware characteristics, so we don't break the API (and that doesn't really matter anyway).

(Review request: http://reviews.vapour.ws/r/5656/)

[natefinch]

$$merge$$

[jujubot]

Status: merge request accepted. Url: http://juju-ci.vapour.ws:8080/job/github-merge-juju

[jujubot]

Build failed: Generating tarball failed
build url: http://juju-ci.vapour.ws:8080/job/github-merge-juju/9243

[natefinch]

hmm, weird timeout with requesting gopkg.in repos... will retry
$$merge$$

[jujubot]

Status: merge request accepted. Url: http://juju-ci.vapour.ws:8080/job/github-merge-juju

[jujubot]

Build failed: Tests failed
build url: http://juju-ci.vapour.ws:8080/job/github-merge-juju/9244

[natefinch]

$$merge$$

[jujubot]

Status: merge request accepted. Url: http://juju-ci.vapour.ws:8080/job/github-merge-juju

[natefinch]

helps if I don't break it
$$merge$$