It looks like that curl tries really hard to set the path to public key and I am afraid that it is not possible to prevent libcurl from doing so:

curl-7.19.7/lib/ssh.c

      home = curl_getenv("HOME");                                                
                                                                                 
      if(data->set.str[STRING_SSH_PUBLIC_KEY])                                   
        sshc->rsa_pub = aprintf("%s", data->set.str[STRING_SSH_PUBLIC_KEY]);     
      else if(home)                                                              
        sshc->rsa_pub = aprintf("%s/.ssh/id_dsa.pub", home);                     
      else                                                                       
        /* as a final resort, try current dir! */                                
        sshc->rsa_pub = strdup("id_dsa.pub"); 

libreport does not explicitly specify the public key path, so libcurl should not use it:

curl-7.45.0/lib/ssh.c

        if(data->set.str[STRING_SSH_PUBLIC_KEY]) {                               
          sshc->rsa_pub = strdup(data->set.str[STRING_SSH_PUBLIC_KEY]);          
          if(!sshc->rsa_pub)                                                     
            out_of_memory = TRUE;                                                
        }

We need the following upstream patch, which is not yet included in RHEL-7:

https://github.com/bagder/curl/commit/curl-7_38_0-47-gfa7d04f

On RHEL-6, we will need also the following upstream commit:

https://github.com/bagder/curl/commit/curl-7_25_0-31-g05a443a

(In reply to Kamil Dudka from comment #11)
> (In reply to Karel Srot from comment #10)
> > Also, 
> > with curl-7.19.7-46.el6 the connection works when scp/sftp --pubkey
> > parameter is omitted. It doesn't work if I pass --pubkey '' instead. I guess
> > this is expected but a confirmation would be appreciated.
> 
> Good point.  It does not seem to work as documented:
> 
> CURLOPT_SSH_PUBLIC_KEYFILE
>     Pass a char * pointing to a file name for your public key.  If not used,
>     libcurl defaults to using ~/.ssh/id_dsa.pub.  (Added in 7.16.1)
>     If an empty string is passed, libcurl will pass no public key to libssh2
>     which then tries to compute it from the private key, this is known to
>     work when libssh2 1.4.0+ is linked against OpenSSL. (Added in 7.25.1)
>
> Surprisingly, it works the same way in the latest upstream.  I will ask them
> whether to update the implementation, or the documentation ... but I would
> rather choose the documentation update for RHEL anyway.

I have notified upstream about this and asked them how they prefer to solve it:

http://thread.gmane.org/gmane.comp.web.curl.library/46793

(In reply to Kamil Dudka from comment #12)
> > ... but I would rather choose the documentation update for RHEL anyway.

Actually, I will rather fix it also in RHEL-6 to make the behavior closer to what we currently have in RHEL-7.

> http://thread.gmane.org/gmane.comp.web.curl.library/46793

upstream commit:

https://github.com/bagder/curl/commit/be538e07

Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory, and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

https://rhn.redhat.com/errata/RHBA-2016-0915.html