Add misspellings handling to "rally info"

Before:
    $ rally info find ExistingServert
    Failed to find any docs for query: 'ExistinfServert'

After:
    $ rally info find ExistinfServert
    Failed to find any docs for query: 'ExistinfServert'
    Did you mean one of these?
        ExistingServers

Two types of misspellings supported as of now:
1. Queries with typos ('ExistinfServert' -> 'ExistingServers')
2. Truncated queries ('Nova' -> 'NovaServers')

We also improve the base.Scenario.get_scenario_by_name() method to ensure
that only scenarios can be returned from it. We simplify the code by adding
the is_scenario() method there.

Change-Id: I093ddef154daeb7cbcd757e35d0e853f70401bee
Merge "Add misspellings handling to "rally info""