kiji-schema-shell: DDLSHELL-69. Add ability to change validation mode of columns from DDL shell

issue: https://jira.kiji.org/browse/DDLSHELL-69
review: https://review.kiji.org/r/1832
commit: https://github.com/kijiproject/kiji-schema-shell/commit/de3b932ec5b3cb59076c732710c88f898eba9fea