Add snapshot retention ops to CLI

[ip1981]

[root@adm ~]# iml snapshot retention create zfsmo 50 g 10
[root@adm ~]# iml snapshot retention list
+----+------------+---------+------+----------+
| Id | Filesystem | Reserve | Keep | Last Run |
+====+============+=========+======+==========+
| 5  | zfsmo      | 50 GiB  | 10   | --       |
+----+------------+---------+------+----------+

[root@adm ~]# iml snapshot retention create zfsmo 10 % 10
[root@adm ~]# iml snapshot retention list
+----+------------+---------+------+----------+
| Id | Filesystem | Reserve | Keep | Last Run |
+====+============+=========+======+==========+
| 5  | zfsmo      | 10 %    | 10   | --       |
+----+------------+---------+------+----------+

Signed-off-by: Igor Pashev pashev.igor@gmail.com

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This change is 

[ip1981]

This is inconsistently optional here and there.

[jgrund]

There is a typo for the SnapshotRetentionPolicies query.