Reduced timeout for SSH connection attempt in tempest.conf

Reduced timeout for SSH connection attempt in tempest.conf

Some tempest test cases try to establish SSH connection to
Cirros VMs using default 300s timeout. Since Cirros VMs send
only three DHCP requests with 1 minute interval, future SSH
connection attempts will most probably fail if DHCP response
will not be received right after third request. For that reason
the optimal timeout is slightly above 2 minutes.

JIRA: FUNCTEST-378

Change-Id: I57415ad37a85cd84887d3cac4f7a44f7e145d820
Signed-off-by: vitikkan <viktor.tikkanen@nokia.com>