Allow to provide custom settings.xml

[lburgazzoli]

Fixes #715
Fixes #706

Support custom settings.xml via configmap/secret as well as some shortcuts to ease the installation process.

apiVersion: camel.apache.org/v1alpha1
kind: IntegrationPlatform
metadata:
  labels:
    app: camel-k
  name: camel-k
spec:
  build:
    maven:
      settings:
        configMapKeyRef:
          key: s.xml
          name: maven-settings

To configure the maven settings at installation time, one can use the --maven-settings option:

    kamel install --maven-settings configmap|secret:name/key    

To quickly set up maven repositories at installation time:

    kamel install --maven-repository http://your/repo@id=something@snapshots@noreleases

NOTES:

• the repository option has been renamed to maven-repository
• the options maven-repository and maven-settings are mutually exclusive
• the repository option is not more available when running an integration

[astefanutti]

Now that we generate the settings file, we may want to handle the local repository the same way.

[lburgazzoli]

I think the problem is still the same, what if the user does not set it ?

[astefanutti]

Right, it's still need a merge strategy, which can be addressed later.

[lburgazzoli]

@astefanutti @nicolaferraro it is now ready for review