zdtm: Switch mnt tests onto new opts

travis-ci: success for mnt: Deprecate --ext-mount-map for --external
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>

test/zdtm/static/mnt_ext_auto.desc

-{'flavor': 'ns uns', 'feature': 'mnt_id', 'opts': '--ext-mount-map auto --enable-external-sharing'}
+{'flavor': 'ns uns', 'feature': 'mnt_id', 'opts': '--external mnt[]:s'}

test/zdtm/static/mnt_ext_manual.desc

-{   'dopts': '--ext-mount-map /zdtm/static/mnt_ext_manual.test:ZDTM',
+{   'dopts': '--external mnt[/zdtm/static/mnt_ext_manual.test]:ZDTM',
     'feature': 'mnt_id',
     'flavor': 'ns uns',
-    'ropts': '--ext-mount-map ZDTM:/tmp/zdtm_ext_auto.XXXXXX/mtest'}
+    'ropts': '--external mnt[ZDTM]:/tmp/zdtm_ext_auto.XXXXXX/mtest'}