(#4424) Add umask parameter to Exec type

Setting a umask value for execs can be useful when the exec creates files or directories.  Relying on the sytem umask can produce unexected results as it may be different from system to system or just not what the user intends..
Merge pull request #1899 from jjulien/feature4424

(#4424) Add umask parameter to Exec type