install-spotify: Update _REPO constants, use wget

Update the REPO_ constants so that both repo, branch and complete
url could be set in environment. Use wget when it's possible to get
things streamlined.
install-spotify, specfile: bugfixes

Don't forget to autoimport gpg keys.  Be flexible with installers
tarball name which can come from any repo. Use rpm to get build
deps from srpm file (not ideal, but there does not seem to be any
other way in suse - yum-builddep is seriosly broken, and zypper si
doesn't work for local packages.).

Add misssing BR: to specfile