Security update for cobbler (moderate)

This update for cobbler fixes the following issues:

The following security issue has been fixed:

• CVE-2017-1000469: Escape shell parameters provided by the user for the
  reposync action. (bsc#1074594)

Additionally, the following non-security issues have been fixed:

• Fix signature for SLES15. (bsc#1075014)
• Detect if there is already another instance of "cobbler sync" running
  and exit with failure if so. (bsc#1081714)
• Add SLES 15 distro profile. (bsc#1090205)
• Require tftp(server) instead of atftp.

This update was imported from the SUSE:SLE-12:Update update project.