[patch][pungi] pungi-splitstages

[Essien Ita Essien]

Hiya,

This is another small patch. What it does is breakup the pungi process 
into different stages that can be invoked independently without going 
thru all the stages. the stages are:

    - Gather (-G)
    - Buildinstall (-B)
    - Package Order (-P)
    - Splittree (-S)
    - Iso Creation (-I)

If none of this switches is specified, then the default behaviour 
continues (i.e, all are run one after the other), specifying *any* of 
the flags will cause the stage to be run.

This split-up actually helps in scenarios when you change something, and 
know what you should be doing next, but do not want to run the whole 
process again. It also helps when debugging the process.

A little issue though is that, I wasn't sure which stage that 
doGetRelnotes should fall under, so I put it under PackageOrdering 
stage, so each time -P is run, GetRelnotes() will also be run.

Cheers,
Essien