Delay Generation of HW Specific Variables

### Problem Description
Currently, we fetch the latest machine data file every time PerfNext is loaded. If some benchmark is selected and the machine data hasn't been fetched yet—fetching that can take a few seconds—then we can get an error while generating HW environment variables.

### Error

```
parser.js: Entering generateHWENV()
/Users/piyush/Work/Git/openjdk-test-tools/PerfNext/app/apis/BenchEngine/parser.js:453
var envVar = HW_ENV[property].$.name;
^

TypeError: Cannot read property 'name' of undefined
at generateHWENV (/Users/piyush/Work/Git/openjdk-test-tools/PerfNext/app/apis/BenchEngine/parser.js:453:38)
at /Users/piyush/Work/Git/openjdk-test-tools/PerfNext/app/apis/BenchEngine/parser.js:393:23
at /Users/piyush/Work/Git/openjdk-test-tools/PerfNext/app/apis/BenchEngine/parser.js:423:9
at FSReqWrap.readFileAfterClose [as oncomplete] (internal/fs/read_file_context.js:53:3)
npm ERR! code ELIFECYCLE
npm ERR! errno 1
npm ERR! PerfNext@0.0.1 start: `node app.js`
npm ERR! Exit status 1
npm ERR!
npm ERR! Failed at the PerfNext@0.0.1 start script.
npm ERR! This is probably not a problem with npm. There is likely additional logging output above.

npm ERR! A complete log of this run can be found in:
npm ERR! /Users/piyush/.npm/_logs/2019-04-01T23_40_41_512Z-debug.log
Piyushs-MacBook-Pro:PerfNext piyush$
```

### Related Snippets

**Frontend: Call to Backend for Getting Machine List**
https://github.com/AdoptOpenJDK/openjdk-test-tools/blob/524d16e3784c17f4af6cee75f9105eb929397792/PerfNext/public/lib/js/benchmarks.js#L34-L40

**Backend: Get machine list**
https://github.com/AdoptOpenJDK/openjdk-test-tools/blob/524d16e3784c17f4af6cee75f9105eb929397792/PerfNext/app/apis/machines.js#L7-L24

**Snippet to Generate HW Specific Variables**
https://github.com/AdoptOpenJDK/openjdk-test-tools/blob/524d16e3784c17f4af6cee75f9105eb929397792/PerfNext/app/apis/BenchEngine/parser.js#L448-L460

### Proposed Changes

We need to find a workaround to prevent this issue.

Start with app/apis/BenchEngine/parser.js around generateHWENV() and trace the machine-list request through app/apis/machines.js and public/lib/js/benchmarks.js. Reproduce startup with a benchmark selected before machine data is available. Done means HW environment generation no longer fails during that fetch and the selected benchmark can continue loading.

Written by the indexing model from the issue text.