Add --ignore-errors parameter to cluster verify

lib/cli.py
- add IGNORE_ERROR_OPT;

client/gnt_cluster.py
- pass the ignore_errors parameter to the opcodes

lib/opcode.py
- update OpClusterVerifyConfig, OpClusterVerify and OpClusterVerifyGroup
  to accept the ignore_errors parameter

lib/cmdlib.py
- pass the ignore_errors parameter to the opcodes that need it

Signed-off-by: Andrea Spadaccini <spadaccio@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>