Ensure generated Step IDs are always valid...

It is possible to generate an invalid ID for a header step when rax:roles Mask is enabled, because the role name is used to construct the id.

The problem is in this function:
https://github.com/rackerlabs/api-checker/blob/api-checker-1.0.17/core/src/main/resources/xsl/raxRolesMask.xsl#L224