This reverts the essence of commit 854e4766:

Changing there

    ${instance-parameter:ipv6-random}

to

    ${instance-parameter:global-ipv6}

was wrong - slapos.cookbook:slapconfiguration recipe, which is leveraged
by [instance-parameter] section does not define global-ipv6:

    http://git.erp5.org/gitweb/slapos.git/blob/HEAD:/slapos/recipe/slapconfiguration.py?js=1#l70

- only ipv6 as "Set of IPv6 addresses (of partition)" and ipv6-random
"One of the IPv6 addresses".

Trying to instantiate a helloworld SR inside webrunner:

    ---- 8< ---- (instance.log)
    ...
    [2015-04-26 11:56:20,133] INFO Error: Referenced option does not exist: instance-parameter global-ipv6
    ...

and the same error while instantiating helloworld SR without slaprunner
under local slapproxy.

~~~~

The global-ipv6 is helpfully injected by slapos.cookbook:softwaretype
recipe into [slap-network-information] section:

    http://git.erp5.org/gitweb/slapos.git/blob/HEAD:/slapos/recipe/softwaretype.py?js=1#l164

but unfortunately only if that slapos.cookbook:softwaretype recipe is
activated - i.e. when we have several software types and we support switching
between them, which is not the case for helloworld (and it's a pity that
such fundamental network-configuration is only optionally provided by SlapOS).

~~~~

So let's switch back to ${instance-parameter:ipv6-random} - this way
helloworld instance can be instantiated again.

Cc: Jérome Perrin <jerome@nexedi.com>