[fix][cli] Pulsar shell: support relative paths (cliextensions)

[nicoloboschi]

Motivation

The pulsar-shell script doesn't change the working directory to the Pulsar home one. In this way if you configure cliExtensionsDirectory with a relative path, it only works if you run pulsar-shell from the pulsar home directory. For instance cd bin && ./pulsar-shell is not able to find the cli extensions dir.

Modifications

• Change directory to pulsar home before starting the java process (in this way it's the same as the other scripts)
  The only disadvantage is that the --file autocompletion will suggest files from the Pulsar home directory and not from
  the real working dir. (but it's still better than the current behaviour)

• doc-not-needed

[eolivelli]

This is tricky.
because with this change all the references to files in pulsar-admin commands will be relative to PULSAR_HOME and not to the current user directory.

The behaviour should be consistent with pulsar-admin when launched from the command line.

Maybe we can do this change (my understanding is that we already do it for pulsar-admin), but we have to enhance pulsar-admin commands that accept relative paths to pick them from the current directory and not from the Pulsar home)

[nicoloboschi]

@eolivelli all the other scripts behave this way

pulsar/bin/pulsar-admin

Line 25 in 98cf15f

#Change to PULSAR_HOME to support relative paths

I agree that is not optimal but it's better to be consistent and do (if needed) the change for each script in the same pull request