Add support for pbuilder --override-config

[bmiklautz]

Some pbuilder settings, like OTHERMIRRORS for example, aren't applied
if update is called without --override-config. To enable this behaviour
the variable OVERRIDE_COWBUILDER_CONFIG was added. If set to true
cowbuilder is run with --override-config.

To make sure that the proper distribution is used, always run cowbuilder
with --distribution.

[bmiklautz]

@mika any change to get this in?

[mika]

Sorry for not responding to this one earlier, beside lack of time I was hold back because of the newly added --distribution "${COWBUILDER_DIST}" and am still not sure about its consequences and what might break in existing setups.

Furthermore I'm reluctant to adding further options (though I fully understand and appreciate your approach to make this configurable!). I'm actually wondering whether there might be any problems with using --override-config by default?

(I looked into testing this today but got sidetracked because of several failing components in jenkins (404-ing plugin files, newly added dependencies,... once again, sigh)

[bmiklautz]

No deal. Don't get stressed with it. Just wanted to check up as it would be really convenient to have this upstream ;).

From my tests I saw that --distribution "${COWBUILDER_DIST}" is required that the correct distribution is used. Without, it happened that when using --override-config I had sources lists from a different distribution.
It might possibly be a good idea to use --override-config per default because when it isn't used it can happen that cow images can't be re-used (like in my case when OTHERMIRRORS were used) but I wasn't sure about any side effects either that's why I added the option.

If you want any adaption/fix of the pull request just let me know.

[mika]

Ok cool :) I'm wondering whether we could come up with a nice test option to verify that the behavior is as intended, maybe we could figure something out together with OTHERMIRRORS usage or so?
(I really need to get some test infrastructure for j-d-g up and running :-/)

[bmiklautz]

For reference pbuilder man page for --override-config

              --distribution  ,  --components  ,  --mirror  , --othermirror options are only valid when
              --override-config option is specified in update target, or when pbuilder create is  being
              called.