Make installation idempotent.

[rhpvorderman]

Hi @edwins ,

First of all thanks for this ansible role. I first tried the other singularity role but it is way more involved, and it depends on golang dep, which is deprecated.

I like that this role is simple and efficient. Because of its simplicity it just works.

One disadvantage I noticed is that the installation always runs, even if singularity is already installed. This slows down deployment, and is especially annoying when you made a small update somewhere else in your playbook or when you are testing. I also noticed that the package installation won't happen on Debian, even though the package names are the same as in Ubuntu.

So with this PR:

• singularity version is run to get the version. (singularity --version when the version is lower than 3). If singularity is not installed, then this command returns an empty string ''.
• The checked version is compared to the desired version.
• If the desired version is not installed the installation runs.

Also:

• Use ansible_os_family because package names among the Debian family and among the RedHat family are interchangable.
• Use state: present for the packages. There is no reason to keep updating the build packages on rerunning the role. Most distributions have only one version in their repo anyway.

[rhpvorderman]

@hexylena. I just notice that you did something similar. Stupid of me not to look. Sorry.

As a note singularity --version prints just the version number on older versions of singularity i.e. 2.x.x. But on singularity 3.x.x singularity --version prints: singularity version 3.x.x. On singularity version 3.x.x singularity version has the desired effect. That is taken into account in this branch, but it could also be applied as a patch to your branch.

[edwins]

@rhpvorderman I apologize for just noticing your pull request and will review it shortly.