Use Terraspace.tmp_root value instead of hard-coded '/tmp/terraspace'

[nevdullcode]

This is a 🐞 bug fix.

• I've added tests (if it's a bug, feature or enhancement)
• I've adjusted the documentation (if it's a feature or enhancement)
• The test suite passes (run bundle exec rspec to verify this)

Summary

Use Terraspace.tmp_root value instead of hard-coded '/tmp/terraspace'.

Context

This resolves the problems I reported here: #123 (comment)

Terraspace.tmp_root was not being used consistently throughout the terraspace library source.

Which are sub-problems of existing reported issue: #123

How to Test

To test, simply run:

export TS_TMP_ROOT=$HOME/test-foo/ts-tmp

Then, run terraspace up <stack> and you should notice all relevant files will be written under the directory specified by the TS_TMP_ROOT environment variable.

Previously, some files would have been written under the directory defined by TS_TMP_ROOT and some would have been written under '/tmp/terraspace' which was not expected, consistent behavior.