Add password input from env variable for occ user:{add, resetpassword}

Add password input from env variable for occ user:{add, resetpassword}

This commit adds the --password-from-env switch to the `occ user:add` and
`occ user:resetpassword` commands. When this parameter is given, Owncloud
will use the password specified in environment variable OC_PASS. This
is safer than using command line parameters, as those can be read by any
process.