Add the GetCACaps operation handling to the SCEP servlet

[borama]

This PR adds the GetCACaps operation to the SCEP servlet. The GetCACaps operation is defined in the new SCEP drafts and it should allow Dogtag to support SCEP clients that require it (or, as an example, we found out that some clients switch to using deprecated hash / encryption algorithms when they don't receive proper response from this operation).

I went through the current SCEP implementation in Dogtag to try to determine which features are exactly supported and which not. If I understood things correctly, Dogtag doesn't support AES in SCEP, certificate renewal (I am about to try to implement this though) and CA cert rollover (the GetNextCACert operation). Other features listed in the draft are supported and printed as the operation response in a simple text form, following the draft specification.

This is the output of calling this operation (using curl):

I am ready for your suggestions, thank you for considering this PR!

Update: I found out that the HTTP POST requests are not properly handled, I opened a separate PR #542 for this.

[ladycfu]

Thank you @borama I think this looks straight forward. I'm holding off on #542 (pending some more tests) for now so I'll temporarily remove "POSTPKIOperation" from the patch before I check in for you.

[fmarco76]

The commit associated on this PR seems already merged d88c48f so I am closing this.