This change allows us to bump the default cirros version
in devstack. Since cirros version 0.6.0 dhcpcd is the default
dhcp client. The older cirros images used udhcpc client (the only
available client at that time) which is also the default client
in Tempest.
This patch makes devstack configure dhcpcd client in tempest.conf
if cirros >= 0.6.0 is going to be used in scenario tests.
The commit also introduces a new SCENARIO_IMAGE_TYPE option.
It is now a trigger for cirros specific settings, later it might
be used for any other image's settings.
Closes-Bug: #2007973
Change-Id: I2738c3b1d302c6656ce2c209671ea954fbc1b05b
a294389403