Adds deployment via snapshot

Adds deployment via snapshot

New arguments are added to allow snapshot deployment:
--snapshot, --snap-cache

The previous tripleo-quickstart code has been removed/replaced
with the snapshot option.

Snapshot deployments are supported on CentOS and Fedora, and snapshot
artifacts use a similar caching system as the standard deployment.
Snapshots are produced daily by Apex, and include latest as well as n-1
OpenStack versions. The os-odl-nofeature scenario is used for the
snapshots. Additionally multiple topology verions of Snapshots are
available. The Snapshot pulled at deploy time depends on the
deploy-settings and number of virtual-computes used at deploy time.

Since there is only one network used with snapshot deployments (admin),
there is no reason to pass in network settings for snapshot deployments.
That argument is now optional. Previously we required even in Standard
virtual deployments that the network settings be provided. However that
is also unnecessary, as we can default to the virtual network settings.

Includes minor fix to the tox.ini to allow specifying test cases
to run (useful for developers writing tests). Default behavior of
tox is unchanged.

JIRA: APEX-548

Change-Id: I1e08c4e54eac5aae99921f61ab7f69693ed12b47
Signed-off-by: Tim Rozet <trozet@redhat.com>