mon,auth,common: include necessary headers

mon,auth,common: include necessary headers

we cannot assume the included header alway indirectly include the use types

in this case, CephContext is defined by ceph_context.h, and
g_ceph_context is declared by global_context.h. so we need to included
them respectively.

and remove unused headers.

Signed-off-by: Kefu Chai <kchai@redhat.com>