Bug 1032978 - Added event_group, chunkNum and chunkTotal to build properties in process/release.py, r=bhearsum

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extra : amend_source : ff59acb8fbe306aa042efd7551c117215c6e9479
Bug 1049099 Thunderbird Linux and Mac MozMill builders are using the system python. r=nthomas
bug 1049595: cope with signing server transition in buildbot configs. r=rail/catlee
Bug 1050109 - stop jobs from hg'n mozharness/tools repos, r=catlee
Bug 1055777 - enable spot instances for some release builders. r=bhearsum
backouts bug: 1050109 - rev: 0a3906d2bffe
merges backout 0a3906d2bffe -> default