[system-tests] fuel-devops Node object has wrong method Node._close_remotes()

Fuel-devops shouldn't track the content of the environment nodes from the base objects like Node, that manage the node itself (hardware/virtual configuration, power status, etc), but *not* it's content.

The patch [1] introduced managing the devops node's content without possibility to get correct IP address associated with the node, see the comment in [2] for details why it is impossible.

To manage the content of the environment nodes, there should be introduced an additional logic layer, like Devops Client [3], and created logical objects like NailgunNodes, that should provide the SSHClient methods to specified networks or instances; provide mapping between fuel-devops Nodes and Nailgun nodes, keep Ip addresses associated with nodes and so on. This NailgunNodes can be inherited by system tests for further purposes.

The current proposal:
1. remove _close_remotes() from devops/models/node.py
2. in fuel-qa repository, clear all connections in Environment.snapshot() and Environment.revert() methods, and close necessary connections from tests if they establish connections not to the devops nodes, but to OpenStack instances or VIP addresses.
3. Add a timer to fuel-devops class _MemorizedSSH() that should close connections if they wasn't accessed for some time. The best profit from memorized connections is in the wait() methods that periodically use some connection to check some data. Each access to the connection can refresh the timer for the connection. If the timer expires, connection will close by _MemorizedSSH() class, and in case of a new access to the same connection it will open a new one.

[1] https://review.openstack.org/#/c/324389/
[2] https://review.openstack.org/#/c/331120/3/devops/models/node.py
[3] https://review.openstack.org/#/c/315999/