Fix configuring mirror for CentOS in nodepool
Fix configuring mirror for CentOS in nodepool

  * add NODEPOOL_CENTOS_MIRROR to set custom CentOS mirror in same manner
    as for Ubuntu
  * add NODEPOOL_EPEL_MIRROR to set custom EPEL mirror
  * use $releasever for CentOS and EPEL repositories

Change-Id: I93c92aea60725bb9b172e6fed12655e4a94b265b

Tesora/tesora-project-config,dongwenjuan/project-config,openstack-infra/project-config,Tesora/tesora-project-config,openstack-infra/project-config,dongwenjuan/project-config