Only set pulsar envs if not already set in the environment.

[cckellogg]

Motivation

Allow the setting of pulsar environment variables from the environment. This makes it easier to configure pulsar in container environments and reduces the need to use a script to do a search and replace on environment variables.

Modifications

Only set pulsar environment variables if they are not already set in the pulsar_env.sh file. Apply the PULSAR_MEM and PULSAR_GC in the pulsar bin file.

Does this pull request potentially affect one of the following parts:

If yes was chosen, please highlight the changes

• Dependencies (does it add or upgrade a dependency): ( no)
• The public API: ( no)
• The schema: (no)
• The default values of configurations: ( no)
• The wire protocol: (no)
• The rest endpoints: (no)
• The admin cli options: (no)
• Anything that affects deployment: (no)

Documentation

• Does this pull request introduce a new feature? (yes / no)
• If yes, how is the feature documented? (not applicable / docs / JavaDocs / not documented)
• If a feature is not applicable for documentation, explain why?
• If a feature is not documented yet in this PR, please create a followup issue for adding the documentation