ci/deploy.sh, lib.sh: Align SSH_KEY with Jenkins

ci/deploy.sh, lib.sh: Align SSH_KEY with Jenkins

Jenkins defines a build param also called SSH_KEY, which holds
the full path to our MCP private RSA key, as opposed to Fuel@OPNFV,
which uses the file basename as SSH_KEY.
Switch to using full path in SSH_KEY, while also moving the key
outside /tmp.

NOTE: When running `ci/deploy.sh` with sudo, key will land in
/root/opnfv/mcp.rsa unless SSH_KEY is set via env var.

While at it, bring back VM image storage dir param, allowing us
to drop one more reference to /tmp.

JIRA: FUEL-280

Change-Id: Id1cfa6dbe7b2e6f3915b22281da5957333718401
Signed-off-by: Alexandru Avadanii <Alexandru.Avadanii@enea.com>