generate-svn-snapshot: support $SBP_OPTS to control svn-buildpackage behaviour + add -nc to its default options
generate-svn-snapshot: support $SBP_OPTS to control svn-buildpackage behaviour + add -nc to its default options

Instead of hardcoding the options to be used with svn-buildpackage
let's support the $SBP_OPTS environment variable, similar to what
we do with $GBP_OPTS in generate-git-snapshot.

We also use `-nc` ("Do not clean the source tree") in
generate-git-snapshot's default $GBP_OPTS, so it makes sense to
use that also in the defaults of generate-svn-snapshot.

Thanks: Christian Schneider <2314b2e3a4a1f7db165be2aafbf1efd78f28cc97@chrisn.dtdns.net> for the patch

hashar/jenkins-debian-glue,webtrekk-kane/jenkins-debian-glue,tonin/jenkins-debian-glue,jbfavre/jenkins-debian-glue,linuxmaniac/jenkins-debian-glue,bmiklautz/jenkins-debian-glue,kartikm/jenkins-debian-glue,linuxmaniac/jenkins-debian-glue,n054/jenkins-debian-glue,mika/jenkins-debian-glue,bmiklautz/jenkins-debian-glue,n054/jenkins-debian-glue,cclausen/jenkins-debian-glue,hetznerZA/jenkins-debian-glue,mika/jenkins-debian-glue,wikimedia/operations-debs-jenkins-debian-glue,kartikm/jenkins-debian-glue,hashar/jenkins-debian-glue,wikimedia/operations-debs-jenkins-debian-glue,webtrekk-kane/jenkins-debian-glue,cclausen/jenkins-debian-glue,hetznerZA/jenkins-debian-glue,jbfavre/jenkins-debian-glue,tonin/jenkins-debian-glue