Ignore extra output when comparing versions with zypper

[nelsonkopliku]

This PR makes sure to ignore extra output of the zypper versioncmp command so that we can only take into account the actual comparinson result.

ie when the /etc/products.d/baseproduct symlink is missing a warning is returned in the output of a version comparison, breaking the parsing of the comparison value (expected to be -1, 0, 1)

$ zypper --terse versioncmp 1.5.2 1.5.2
Warning: The /etc/products.d/baseproduct symlink is dangling or missing!
The link must point to your core products .prod file in /etc/products.d.

0

[dottorblaster]

I couldn't help asking myself "what if we get an empty string?"

but still, LGTM