modified startup code to only set mock group [BZ# 809676]
    
      - add CLONE_NEWUTS to unshare(2) call [BZ# 818445]
      - from Seth Vidal <skvidal at fedoraproject.org>:
        - add mockchain to mock [BZ# 812477]
      - from Marko Myllynen <myllynen@redhat.com>:
        - fix write_tar check in scm.py [BZ# 828677]
      - from Masatake YAMATO <yamato@redhat.com>:
        - added option to set a plugin parameter value [BZ# 754321]