Changes

ttcn3-*/jenkins*.sh: use $SUBNET everywhere

Use the SUBNET variable instead of hardcoding it in some places. Split
commands across multiple lines while at it to improve readability.

Related: OS#5802
Change-Id: I08f83089ef97f5f92d4bbfa5743301e7375e9f0f

network_create: find free subnet automatically

Try multiple subnet numbers until successfully creating a network. This
way we can run the same ttcn3 testsuite multiple times in parallel
without conflicts (e.g. once against latest, once against nightly). Also
we don't need to make sure each new testsuite has a unique subnet
number anymore.

I've considered also adjusting network_bridge_create, but that gets used
exclusively by osmo-ran/jenkins.sh, a script which we don't actually run
in jenkins. It seems that in this script it makes more sense to not get
a random subnet number.

Related: OS#5802
Change-Id: I57152b08ef0f38e17e7019a8df032189b03f56cf