Install from latest release instead of master

[AMeng]

Make the install script idempotent by locking in the latest version number. This value needs to be updated each time a new release is pushed out.

An alternative solution would be to pass in the release version as an argument to this script. I think even if this option was to be added, it should still default to the release from which the install script was taken.

Without this change, the install script cannot reliably be used for idempotent builds.

[michaelwittig]

Hi @AMeng

We only have versioned RPMs at the moment.

I don't like to hardcode the version because I'm the one who has to keep the version in sync.

As you suggested, a argument to the install script that defaults to master would be much better. Would you be able to make the change in your PR?

[AMeng]

Added an option for specifying the release which defaults to master. The new help text looks like this:

$ ./install.sh -h
Usage: install.sh [-hv] [-a ARN] [-i GROUP,GROUP,...] [-l GROUP,GROUP,...] [-s GROUP] [-p PROGRAM] [-u "ARGUMENTS"] [-r RELEASE]
Install import_users.sh and authorized_key_commands.

    -h                 display this help and exit
    -v                 verbose mode.

    -a arn             Assume a role before contacting AWS IAM to get users and keys.
                       This can be used if you define your users in one AWS account, while the EC2
                       instance you use this script runs in another.
    -i group,group     Which IAM groups have access to this instance
                       Comma seperated list of IAM groups. Leave empty for all available IAM users
    -l group,group     Give the users these local UNIX groups
                       Comma seperated list
    -s group,group     Specify IAM group(s) for users who should be given sudo privileges, or leave
                       empty to not change sudo access, or give it the value '##ALL##' to have all
                       users be given sudo rights.
                       Comma seperated list
    -p program         Specify your useradd program to use.
                       Defaults to '/usr/sbin/useradd'
    -u "useradd args"  Specify arguments to use with useradd.
                       Defaults to '--create-home --shell /bin/bash'
    -r release         Specify a release of aws-ec2-ssh to download from GitHub. This argument is
                       passed to `git clone -b` and so works with branches and tags.
                       Defaults to 'master'