Changes

jenkins-common: tweak echo messages

Use 'set +x' before echo, so messages are not printed twice and the
output becomes more readable.

Example:
  + echo Creating network ttcn3-bts-test
  Creating network ttcn3-bts-test

Related: OS#5802
Change-Id: I213cb80bd1d9401281ae1fb7394b2319453550dd

ttcn3-ggsn-test: cp initrd-ggsn.sh to VOL_BASE_DIR

Prepare for a future patch where a free subnet will be figured out
automatically instead of having a hardcoded one, by moving
initrd-ggsn.sh to $VOL_BASE_DIR as the subnet will be replaced in files
in this directory.

Related: OS#5802
Change-Id: Ie4f35b77a2bff20f2a0479890a429754768cafd2

scripts/regen_doc: use SUBNET variable

Add a $SUBNET parameter and use it throughout the script, similar to
other ttcn3-*/jenkins.sh scripts. This is in preparation for figuring
out a free network in network_create() automatically in a future patch.

Related: OS#5802
Change-Id: I3bda4bd7280469ad5ee02104295134a2e0b768e3