zebra: Disable mpls slightly different

zebra: Disable mpls slightly different

When mpls is not turned on in the kernel, we
are not installing the mpls commands into the cli.
This results in vtysh attempting to run the command
and receiving a 'WTF is this command' back from zebra.
Modify the mpls code to install commands and to check
to see if the command should be accepted based
upon mpls working or not.

Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>
Reviewed-by: Daniel Walton <dwalton@cumulusnetworks.com>