rapi.baserlib: Add function to check variable type
rapi.baserlib: Add function to check variable type

Also add a separate function to retrieve body parameters. Having
it separate (independent of a class instance) will make it easier
to unittest users of this function.

Signed-off-by: Michael Hanselmann <68b67885ff74281219b4a4be33cc20fd45078d7c@google.com>
Reviewed-by: Guido Trotter <f62ca4d5358bdaeef0e50e57f9f7e231de63727e@google.com>

ganeti/ganeti,mbakke/ganeti,mbakke/ganeti,ganeti/ganeti,leshchevds/ganeti,apyrgio/ganeti,ganeti-github-testing/ganeti-test-1,bitemyapp/ganeti,yiannist/ganeti,dimara/ganeti,leshchevds/ganeti,dimara/ganeti,onponomarev/ganeti,yiannist/ganeti,apyrgio/ganeti,bitemyapp/ganeti,onponomarev/ganeti,yiannist/ganeti,apyrgio/snf-ganeti,ganeti-github-testing/ganeti-test-1,ganeti/ganeti,mbakke/ganeti,andir/ganeti,leshchevds/ganeti,grnet/snf-ganeti,grnet/snf-ganeti,andir/ganeti,apyrgio/snf-ganeti,andir/ganeti