A proposed fix for bug #1384622

Added a check to the "juju set" command, which issues a warning to the user
if the value of the configuration option is already set to the desired value.

The check is done by retrieving the unit's configuration options via an
additional API call and comparing the user-defined value with the
existing configuration value.
Added a test scenario that asserts the warning when attempting to set a config option to the existing value
Changed wording of the warning and modified the tests
Merge pull request #958 from alesstimec/bug#1384622

Bug #1384622 - juju set should give feeback that the value is already set

Added a check to the "juju set" command, which issues a warning to the user
if the value of the configuration option is already set to the desired value.

The check is done by retrieving the unit's configuration options via an
additional API call and comparing the user-defined value with the
existing configuration value.