Hide the --local flag of the "plugin install" cmd

[marckhouzam]

What this PR does / why we need it

Hide the --local flag of the "plugin install" cmd to avoid giving the impression that it can be used in production.
The plugin install --local form should only be used when doing local testing while developing a plugin and it should be used implicitly through make plugin-install-local that is introduced in #139 .

Which issue(s) this PR fixes

Fixes #152

Describe testing done for PR

# Notice there is no --local flag shown
$ tz plugin install -h

# Notice there is no shell completion of the --local flag
$ tz plugin install builder --loc<TAB>

# Notice the --local flag still works even though it is hidden
$ tz plugin install builder --local ./artifacts/plugins/darwin/amd64/
[i] Installing plugin 'builder:v0.1.0-dev-23-g23f63f168' with target 'global'
[ok] successfully installed 'builder' plugin

Release note

The "tanzu plugin install --local" form is no longer to be used in production.  The "--local" flag is therefore hidden.

Additional information

Special notes for your reviewer

[anujc25]

LGTM