Global Update Policy

See https://issues.apache.org/jira/browse/KARAF-2183

This pull request introduces new configuration property
```
org.ops4j.pax.url.mvn.globalUpdatePolicy
```
which can accept ```updatePolicy``` values
http://maven.apache.org/settings.html#Repositories
```
updatePolicy: This element specifies how often updates should attempt to occur. Maven will compare the local POM's timestamp (stored in a repository's maven-metadata file) to the remote. The choices are: always, daily (default), interval:X (where X is an integer in minutes) or never.
```

When provided value is "always":
```
org.ops4j.pax.url.mvn.globalUpdatePolicy = always 
```
Then pax-url-aether will always resolve latest snapshot from remote maven repository.

This is helpful during development, when dependencies are snapshots.