add engine-strict flag to npm install args

[nileio]

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

Proposed changes

This PR adds --engine-strict flag to the npm install command for installing custom nodes. This flag ensures that if a specific nodejs version is included under engines section in package.json the npm install command will fail if it does not satisfy the nodejs version requirements rather than only generate a warning message.
to use this feature , a node developer adds an engines section in package.json and specify the version requirements such as
{ "engines" : { "node" : ">=0.11.15" } }

This is tested to check for a specific nodejs version. This could possibly be extended to also stipulate a minimum node-red version which will be very useful to ensure that end users are not installing any incompatible nodes which will prevent node-red to load in some cases [however, using this flag to check for node-red version is not tested]

Checklist

• I have read the contribution guidelines
• For non-bugfix PRs, I have discussed this change on the forum/slack team.
• I have run grunt to verify the unit tests pass
  running grunt failed at 1) externalModules api checkFlowDependencies: I don't know whether the change caused grunt to fail or is it something else?! first PR so I am not sure whether I need to update the unit test or not. Please let me know
• I have added suitable unit tests to cover the new/changed functionality

[knolleary]

Given the failing unit tests are in the corresponding file to one you've modified, then yes, that needs investigating.

I can take a look tomorrow before we merge this.

[knolleary]

Found the issue - the externalModules tests use the module name passed to npm install in order to verify different behaviours. The test assume the module name was the 3rd argument passed in - but --engine-strict added, its now the 4th argument.

I've fixed the tests to look at the last argument so it doesn't cause problems again if we add other default args.