Tree: Expand Button text is hard-coded and cannot be changed by consumer

[interactivellama]

"Expand Tree Branch" text is not able to be changed by library consumers.

I propose updating Tree props to:

	assistiveText: PropTypes.shape({
		label: PropTypes.string,
		expandBranchLabel: PropTypes.string,
	}),

expandBranchLabel default value is "Expand Tree Branch".

https://github.com/salesforce/design-system-react/blob/master/components/tree/index.jsx#L28

[interactivellama]

@AnutaK This bug fix is very close to #2007. A few of the same lines of code get touched, so I thought I'd group it. This is mostly making the line https://github.com/salesforce/design-system-react/blob/master/components/tree/private/render-branch.jsx#L307 a variable and getting that variable from the top level tree/index.jsx props.

Is what I am asking for clear?