Ironic and baremetal install diskimage-builder with pip

This change installs diskimage-builder with pip instead
of from git, and only if the executable ramdisk-image-create
isn't already installed.

This allows the ironic and baremetal to consume the latest
release of diskimage-builder by default, and gives the option of
defining version constraints for which version to install.

If the dib service is enabled (which it isn't by default) then
diskimage-builder will already be installed from git and will
not be re-installed for ironic and diskimage-builder.

Change-Id: Ic6613a11e2c4fb7a1261b81cdc2cf958515e60cd
Merge "Ironic and baremetal install diskimage-builder with pip"