add the ability for retention policy context in cli with use command

[corylanou]

fixes #3188

example usage:

> create database foo
> show retention policies on foo
name    duration shardGroupDuration replicaN default
----    -------- ------------------ -------- -------
autogen 0s       168h0m0s           1        true

> create retention policy bar on foo duration 1h replication 1
> show retention policies on foo
name    duration shardGroupDuration replicaN default
----    -------- ------------------ -------- -------
autogen 0s       168h0m0s           1        true
bar     1h0m0s   1h0m0s             1        false

> use foo
Using database foo
> insert cpu,context=foo value=1
> select * from cpu
name: cpu
time                context value
----                ------- -----
1482248174482948380 foo     1

> use foo.bar
Using database foo
Using retention policy bar
> insert cpu,context=bar value=2
> select * from cpu
name: cpu
time                context value
----                ------- -----
1482248211927258844 bar     2

> settings
Setting           Value
--------          --------
Host              localhost:8086
Username
Database          foo
RetentionPolicy   bar
Pretty            false
Format            column
Write Consistency all

Required for all non-trivial PRs

• Rebased/mergable
• Tests pass
• CHANGELOG.md updated

Required only if applicable

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• InfluxData Documentation: add updated usage for use and new clear commands in cli docs.influxdata.com-ARCHIVE#923

[e-dard]

Just the one nit but LGTM 👍

[e-dard]

nit: you can make this a bit shorter:

for i := 1; i < len(values); i++ {
	if values[i] == rp {
		return true
	}
}