api: Implement log_dump/log_details

api: Implement log_dump/log_details

- Replaces the need to screen scrape "show log".
- Adds an api to return the system time.  When running over a socket, the
  api client may have different time than the vpp host.

expected use:
  vpp_time_before_command = self.vapi.show_vpe_system_time_ticks().vpe_system_time_ticks
  <run some commands>
  log_output = self.vapi.log_dump(start_timestamp=vpp_time_before_command)

Depends-on: https://gerrit.fd.io/r/20484
Depends-on: https://gerrit.fd.io/r/#/c/19581/

==============================================================================
TestVpeApi
==============================================================================
log_details(_0=838, context=3, timestamp_ticks=2.4954863503546518e+48, level=<vl_api_log_level_t.VPE_API_LOG_LEVEL_WARNING: 4>, timestamp=u'2019/07/04 20:35:41:281', msg_class=u'buffer', message=u'vlib_physmem_shared_map_create: clib_mem_create_hugetlb_fd: open: No such file or directory\n\n')
log_details(_0=838, context=3, timestamp_ticks=1.6101902879480125e+159, level=<vl_api_log_level_t.VPE_API_LOG_LEVEL_WARNING: 4>, timestamp=u'2019/07/04 20:35:41:281', msg_class=u'buffer', message=u'falling back to non-hugepage backed buffer pool')
test_log_dump_default (test_vpe_api.TestVpeApi)                          OK
log_details(_0=838, context=13, timestamp_ticks=2.4954863503546518e+48, level=<vl_api_log_level_t.VPE_API_LOG_LEVEL_WARNING: 4>, timestamp=u'2019/07/04 20:35:41:281', msg_class=u'buffer', message=u'vlib_physmem_shared_map_create: clib_mem_create_hugetlb_fd: open: No such file or directory\n\n')
log_details(_0=838, context=13, timestamp_ticks=1.6101902879480125e+159, level=<vl_api_log_level_t.VPE_API_LOG_LEVEL_WARNING: 4>, timestamp=u'2019/07/04 20:35:41:281', msg_class=u'buffer', message=u'falling back to non-hugepage backed buffer pool')
test_log_dump_timestamp_0 (test_vpe_api.TestVpeApi)                      OK
test_log_dump_timestamp_future (test_vpe_api.TestVpeApi)                 SKIP
test_show_vpe_system_time_ticks (test_vpe_api.TestVpeApi)                SKIP

==============================================================================
TEST RESULTS:
     Scheduled tests: 4
      Executed tests: 4
        Passed tests: 2
       Skipped tests: 2
==============================================================================

Test run was successful

Type: feature

Change-Id: I893fc0a65f39749d2091093c2c604659aadd8447
Signed-off-by: Paul Vinciguerra <pvinci@vinciconsulting.com>