Add NodeNotFound() exception to common exceptions

NodeNotFound() raises in db.api get_node() method, if we have
no such node in database, but this excepton was absent in
tuskar.common.exception module.
Added missed exception and tests for get_node() function.

Change-Id: I3193b0dd52a7263903604a17cc2315936f3b5d02
Merge "Add NodeNotFound() exception to common exceptions"