Add support for --releasever

[fcrozat]

This is a follow-up of a twitter thread I wrote when openSUSE Leap 15.2 was released (https://twitter.com/fcrozat/status/1279130917223661568 ):

it is more difficult to migrate openSUSE Leap from one release to another using transactional-update, compared to zypper, due to releasever CLI option flag from zypper not being supported by transactional-update.

I would suggest transactional-update to accept this flag as additional option and pass it to zypper transparently.

[wombelix]

I used the Environment Variable "ZYPP_REPO_RELEASEVER" instead the "--releasever" Parameter as Workaround and could successfuly update from Leap 15.1 to 15.2 that way.

sudo su -
export ZYPP_REPO_RELEASEVER=15.2
transactional-update dup