Add missing __contains__() and get() methods to QuotaUsage

The QuotaUsage class was missing __contains__() and get() methods which was
causing unexpected behaviour, e.g:

    >>> usages = quotas.tenant_quota_usages(request)
    >>> 'ram' in usages
    False
    >>> usages.get('ram')
    None

Change-Id: Ib0b1ad69c4bf4effd7c10827d23bcc713bdc5ed6
Merge "Add missing __contains__() and get() methods to QuotaUsage"