Add snapshot interval operations to CLI

[ip1981]

[root@adm ~]# iml snapshot interval list
+----+------------+---------------------+-------------+----------+
| Id | Filesystem | Interval            | Use Barrier | Last Run |
+====+============+=====================+=============+==========+
| 1  | zfsmo      | 1 minute            | false       | --       |
+----+------------+---------------------+-------------+----------+
| 3  | zfsmo      | 1 week              | false       | --       |
+----+------------+---------------------+-------------+----------+
| 4  | zfsmo      | 1 year and 11 hours | false       | --       |
+----+------------+---------------------+-------------+----------+

[root@adm ~]# iml snapshot interval add zfsmo  10 year -b
[root@adm ~]# iml snapshot interval list
+----+------------+-------------------------------+-------------+----------+
| Id | Filesystem | Interval                      | Use Barrier | Last Run |
+====+============+===============================+=============+==========+
| 1  | zfsmo      | 1 minute                      | false       | --       |
+----+------------+-------------------------------+-------------+----------+
| 3  | zfsmo      | 1 week                        | false       | --       |
+----+------------+-------------------------------+-------------+----------+
| 4  | zfsmo      | 1 year and 11 hours           | false       | --       |
+----+------------+-------------------------------+-------------+----------+
| 10 | zfsmo      | 10 years, 2 days and 12 hours | true        | --       |
+----+------------+-------------------------------+-------------+----------+

[root@adm ~]# iml snapshot interval remove 1 3
[root@adm ~]# iml snapshot interval list
+----+------------+-------------------------------+-------------+----------+
| Id | Filesystem | Interval                      | Use Barrier | Last Run |
+====+============+===============================+=============+==========+
| 4  | zfsmo      | 1 year and 11 hours           | false       | --       |
+----+------------+-------------------------------+-------------+----------+
| 10 | zfsmo      | 10 years, 2 days and 12 hours | true        | --       |
+----+------------+-------------------------------+-------------+----------+

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

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This change is 

[jgrund]

Just a comment on the conflict change, otherwise LGTM.

[mkpankov]

Reviewed 5 of 7 files at r1, 2 of 2 files at r2.
Reviewable status: all files reviewed, 2 unresolved discussions (waiting on @ip1981)

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iml-manager-cli/src/snapshot.rs, line 28 at r2 (raw file):

        #[structopt(short = "d", long = "display", default_value = "tabular")]
        display_type: DisplayType,
        // TODO? Optional FS?

Still TODO?

[jgrund]

nit: clippy warning here.