mon: MonCommands: have all 'auth' commands require 'execute' caps

Earlier patch already have the entity requiring 'execute' caps for
read-only commands.  This patch introduces the same requirement for *all*
auth commands, read-only and read-write alike.

While the rationale behind the earlier patch for leaving read-write
operations out of this requirement still holds, we now enforce this to
match compatibility with what was happening back on Dumpling with regard
to the 'execute' cap being required for auth commands.  However, it should
be noted that back on Dumpling we were only requiring the 'execute' cap
for auth commands, regardless of read-only or read-write, and no other
caps were required.

Fixes: 7919

Patch series also add a workunit to test all combinations of caps and make
the mon behaves as expected.

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>