Make it possible to configure bundle dirs

[rlenferink]

Currently bundles specified in the cosgi.auto.start.1 config property should be a) absolute path or relative paths from the work dir.

Extend the framework so that multiple bundle dirs can be specified to search for existing bundles using a config property. If this property is not set, the result should be that there is a single bundle dir configured to the work dir. Multiple dirs can be specified using using colons.

Proposed config property: CELIX_BUNDLE_DIRS

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Original creation date: 09/Jan/2018 Reporter: @pnoltes