Check stats collecting after tests

Check that usage stats are sent (if sending enabled)
or not sent (if sending disabled) after the following
tests:

 - deploy_flat_stop_reset_on_deploying
 - deploy_reset_on_ready
 - multiple_cluster_net_neutron_gre_ha

In system tests we can use one environment (with the
same master node uuid) in several different tests,
so the count of sent to collector logs can be greater
than we expect in particular test.

Also, improve 'get_ssh_to_remote_by_key' method,
so it can get SSH keys from SSH-Agent (usefull
on Jenkins) and fail test if Fuel master node is
unavailable via SSH after resuming environment.

Change-Id: I90d3c6f77f303629e2eeaf6d12b82da0f2510140
Implements: blueprint fuel-stats-test