Permit to install from experimental repository or snashot

[clarsonneur]

jplugins should be able to

• download from experimental for some plugins
• re-use an existing plugins installed like a snapshot file

[clarsonneur]

There is a jenkins mechanism to download some plugins by commit IDs.

Ref: https://jenkins.io/blog/2018/05/15/incremental-deployment/

Plugins in such use case is quite limited. Look in https://repo.jenkins-ci.org/incrementals/org/jenkins-ci/plugins/

Jenkinsci/docker repo implements this as follow:
https://github.com/jenkinsci/docker/blob/master/install-plugins.sh#L72

We should follow the same rule and reproduce the install-plugin.sh behavior.