cobbler.spec - do not require tftp-server

Cobbler can use dnsmasq's tftp server, or in.tftpd, or the tftpd
provdided with Cobbler itself. Depending upon the configuration,
tftp-server may not be used and therefore its installation should
not be required.
Merge pull request #696 from mark987/no-tftp-spec

cobbler.spec - do not require tftp-server