[packman] SLE_12 starts to build against SP1

[Olaf Hering]

In OBS the binary project "SUSE:SLE-12-SP1:GA" was finally populated on
2016-02-02. Today I switched the SLE_12 repo to build against this
project, instead of "SUSE:SLE-12:GA". It will most likely take the rest
of the week until packman has settled.

I dont expect any incompatiblities from this change, even if used with
SLE12-GA. 'zypper dup -r packman' remains the way to install and upgrade
packages from packman.

Olaf