Chutney 'bootstrap-network.sh' shouldn't overwrite 'CHUTNEY_DATA_DIR'
When you set up a tor network with something like:
```
tools/test-network.sh --flavor basic --net-dir /tmp/chutney-net
```
the `tools/test-network.sh` script will set `$CHUTNEY_DATA_DIR` based on the value of `--net-dir` and then call `tools/bootstrap-network.sh`. This bootstrapping script will then overwrite the variable `$CHUTNEY_DATA_DIR` if that directory doesn't exist or if the path is relative. This causes unexpected behavior when the user explicitly sets the `--net-dir` option. For example, Chutney works fine if you provide it with a directory that doesn't exist (it will create that directory automatically), so there is no need for the `tools/bootstrap-network.sh` script to unexpectedly change it to `$CHUTNEY_PATH/net`. The `tools/bootstrap-network.sh` also does not need to change it to an absolute path since Chutney [does this automatically](https://github.com/torproject/chutney/blob/master/lib/chutney/TorNet.py#L144).
My suggestion is to remove the following lines from `tools/bootstrap-network.sh`:
```
# Get a working net path
if [ ! -d "$CHUTNEY_DATA_DIR" ]; then
# looks like a broken path: use the chutney path as a base
export CHUTNEY_DATA_DIR="$CHUTNEY_PATH/net"
fi
if [ -d "$PWD/$CHUTNEY_DATA_DIR" ]; then
# looks like a relative path: make chutney path absolute
export CHUTNEY_DATA_DIR="$PWD/$CHUTNEY_DATA_DIR"
fi
```
issue