Ability to set the source version to 'latest-snapshot'

[rs017991]

The existing 'latest' behaviour only works for releases, so I added a 'latest-snapshot' option which will result in the latest unique version of the latest snapshot.

Examples

Let's say you have the following unique versions of your artifact:
1.0.0-SNAPSHOT:

• 1.0.0-20170328.031519-19
• 1.0.0-20180429.041620-20

2.0.0-SNAPSHOT:

• 2.0.0-20190101.010101-01
• 2.0.0-20190202.020202-02

Then these are the results you will get by setting the source.version to the following values:

source.version	Resulting JSON
1.0.0-SNAPSHOT	[{"version": "1.0.0-20180429.041620-20"}]
2.0.0-SNAPSHOT	[{"version": "2.0.0-20190202.020202-02"}]
latest-snapshot	[{"version": "2.0.0-20190202.020202-02"}]

[rs017991]

@patrickcrocker Note: Everything in the test folder is g2g, but I wasn't able to run itest so didn't feel comfortable changing those (aside from constant that I moved to helpers.sh)

[rs017991]

BTW if anybody wants to try this out, I have it deployed to Dockerhub:

resource_types:
- name: maven-resource
  type: docker-image
  source:
    repository: rs017991/maven-resource
    tag: PR34

That said, I'll likely delete it after merge, so don't go depending on it for real.