add 'auto' query parameter to 'slp' url scheme (bsc#1213531)

[wfeldt]

Task

• https://bugzilla.suse.com/show_bug.cgi?id=1213531

Skip interactive dialog presenting a list of matching SLP entries and automatically use the first entry.

For example:

# choose first installation source with 'tumbleweed' in its description
install=slp:?descr=*tumbleweed*&auto=1