Add structure, unit tests, and template for mistral-specs, creating
a functional specs repository without any approved specs.
Put example.rst in mitaka/approved and mitaka/implemented respectively
to avoid doc build warnings:
"pattern u'specs/mitaka/approved/*' didn't match any documents"
Implements: blueprint mistral-spec-repository
Change-Id: I092ec2f9b1aef9b1bf328658674119eb3cdf3380