Added property osname to support AIX rpm.

[pradeepbansal00]

This is simply the porting of change which Brian Cox did on v3.2.1 version
( https://github.com/nebula-plugins/gradle-ospackage-plugin/compare/master...brc0x1:feature/osname?expand=1)

[coveralls]

Coverage decreased (-0.1%) to 85.388% when pulling b78f71f on pradeepbansal00:v3.6.1 into 1bd9961 on nebula-plugins:master.

[aaltman]

Looks good, thanks @pradeepbansal00.

[mnellemann]

Is there any way this can be merged ?

[chali]

Thank you for the contribution. Could you rebase the PR?

[mnellemann]

It looks like a lot of changes has been been made since this PR.

I have done some testing (id "nebula.ospackage" version "8.5.6") and tried to build an rpm for AIX with the following task in my build.gradle:

task buildRpmAix(type: Rpm) {
    dependsOn build, startShadowScripts
    packageName = 'jload-AIX'
    os = Os.AIX
    arch = Architecture.NOARCH
    type = RpmType.BINARY
}

When I try to install it on AIX 7.2, I get:

# rpm -i jload-AIX-1.1.3-1.noarch.rpm
        package jload-AIX-0:1.1.3-1.noarch is intended for a different operating system

I can use the --ignoreos flag, and it installs just fine (as a noarch package). But this I can also do with the rpm package build with os = Os.LINUX. Not sure what to make of this. My skills are not up to doing any major code changes, but I would like test or help otherwise.