Fix plugin script to allow spaces in ES_HOME

[nik9000]

Fixes ES_HOME with spaces and installing plugins from a local directory
with spaces.

Closes #12504

[nik9000]

This is pretty much a clean forward port of #12544. It was difficult to test because the tests require a 2.0 compatible version of watcher which you have to build from source. And to do that you have to be past of the Elastic organization because that is a closed source plugin. That's certainly going to have to change.

Ping @tlrx, @spinscale, and @electrical who looked at the 1.7 version of this.

[electrical]

Looks good to me as far as i can see.

[tlrx]

It was difficult to test because the tests require a 2.0 compatible version of watcher which you have to build from source.

Are you talking about the BATS tests? If so, maybe we should move them apart.

[jaymode]

LGTM

[nik9000]

Are you talking about the BATS tests? If so, maybe we should move them apart.

They already have this problem because the plugin installer tries to fetch watch for 1.7 and can't. Since it isn't making anything worse I'll just merge this as is.

I'll create a followup issues to the bats tests not being consistent.