Bugzilla: pass Bugzilla_token in all XML RPC calls

Introduce the session parameters for XML RPC calls. These parameters are
added to every XML RPC call.

abrt_xmlrpc_call*() functions expected formatting string in form of
"({...})" for some good but unknown reason. Since now, the functions
expects formatting string without the outer brackets.

() - means empty array (allowed in xmlrpc-c)
{} - means empty structure (allowed in xmlrpc-c)

Cite:

Instead of returning a cookie, the User.login call now returns a token
that clients must pass in the Bugzilla_token parameter to subsequent
RPC calls. If the token is not passed, Bugzilla will treat the RPC
call as unauthenticated and will not allow access to non-public data.

See
https://partner-bugzilla.redhat.com/docs/en/html/api/Bugzilla/WebService.html#LOGGING_IN
for more details.

Client scripts that access Red Hat Bugzilla via XML-RPC or JSON-RPC
and use login cookies for authentication must be updated to instead
remember the token received when logging in and pass that token back
to Bugzilla in subsequent RPC calls.

[http://post-office.corp.redhat.com/archives/bugzilla-list/2014-April/msg00005.html]

Resolves rhbz#1090466

Signed-off-by: Jakub Filak <jfilak@redhat.com>