Load execution context from file when creating execution via CLI

When starting a new workflow execution using CLI we should be able to load execution context from a file. Now we get:

bash-3.2$ mistral execution-create my_wb execute_backup my_ctx.json
usage: mistral execution-create [-h] [-f {shell,table}] [-c COLUMN]
                                [--variable VARIABLE] [--prefix PREFIX]
                                workbook task context
mistral execution-create: error: argument context: invalid loads value: 'my_ctx.json'

which means we can only pass values as a string.