[BACKPORT - #227] Check if parameter is already declared to avoid re-declaring it

[bergercookie]

This is a backport of PR #227 to be used in galactic

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A parameter can only be declared once. When implementing a LicecycleNode
one would normally do all the initialization in on_configure() and the
cleanup, deinitialization in the on_cleanup method. Furthermore, these
transitions could potentially be called multiple times, depending on how
the node is t ransitioned from the lifecycles are triggered. However,
this approach doesn't currently work well with Diagnostics since, in the
constructor, of diagnostic_updater::Updater it's declaring a static
parameter without checking whether that parameter has already been
declared. In subsequent calls to on_configure (and thus,
if Diagnostics are initialized there, in subsequent calls to the
Updater constructor, this results in the following message:

Original error: parameter 'diagnostic_updater.period' has already been declared

To avoid this, until the Foxy version, the wrapper code could explicitly
call undeclare_parameter on the parameter that the Updater is
declaring, but as of Galactic, undeclaring a static parameter is not
allowed anymore. See e.g., ros2/rclcpp#1850.

In this commit, I'm checking explicitly if the parameter has already
been registered and if so, I'm just reading its value before attempting
to re-declare it.

[com2rng]

Same comment as in #227