Implemented build of default instance for union types.

Implemented build of default instance for union types.

For each union type class <union-name>Builder will be generated to folder specified in persistentSourcesDir parameter in build configuration of yang-maven-plugin.
If parameter is not specified, this builder will be generated to src/main/java by default. Each such builder will contain getDefaultInstance method, which accepts String parameter.
This method will be called from union class constuctor with char[] argument and creates default instance for given union type.

Implemented method getParamNameFromType to create param name from given TypeDefinition.
Updated tests.

Change-Id: I78ebe37acd732fd70c6fe152c18c6c91224cb5e4
Signed-off-by: Martin Vitez <mvitez@cisco.com>