execute mvn with maven.settingsFile

[emliunix]

Re-create PR #768 because it's accidentally closed.

Original description:

Now that we have maven.settingsFile config, it seems natural to execute mvn with it (which I found out not.)
Is there any concern not doing so.

[ghost]

All CLA requirements met.

[Eskibear]

LGTM

[Eskibear]

@jdneo
To verify:

1. open a Maven project.
2. modify setting maven.settingsFile and point it to a valid settings.xml
3. run a maven goal, verify if -s settings.xml is appended to the command.
4. create a new project, also verify command line in the task.