1133953 - check Mongo version during startup

Previous versions of Pulp would not check the version of the Mongo database
during startup. If the version of Mongo was older than what Pulp expected,
issues would arise during runtime that could be difficult to debug.

Instead, check the Mongo version at startup and fail immediately if it's not
compatible with Pulp.

https://github.com/mongodb/mongo/blob/master/src/mongo/util/version.cpp
contains information about how Mongo versioning works.
self.fail() could be used for this to avoid the assertTrue(False) idiom. Also it could be moved to an else block so that it runs only if an Exception is not encountered.