SA: Fix private key file name generation issue

SA: Fix private key file name generation issue

In current implementation of Yardstick, SA context uses the
same SSH key file name for each VM created. E.g. the Yardstick
overrides the key file context each time the new VM is created.
As a result, the key file is valid only for last VM created
by Yardstick causing the connection problem for other VMs.
Solution is to generate different key file name for each VM
name separately.

JIRA: YARDSTICK-1436

Change-Id: I22ae2874695736255aa3314654a2cced9338b53d
Signed-off-by: Mytnyk, Volodymyr <volodymyrx.mytnyk@intel.com>