Setting default network bridge name on config creation.
Review fix.
Review fix.
Moved setting of defaults to Validate.
Merge pull request #1196 from anastasiamac/Bug1394450-older-client-destroing-new-environment

Setting default network bridge name on config creation.

Older clients can't destroy newer environments because a) default network bridge name changed to empty string and b) reasonable default for network bridge name was only provided via getter.

This change moves the guess of reasonable default to config creation. 

(Review request: http://reviews.vapour.ws/r/507/)