layman_.py: Adds re-initialization prevention code

This commit prevents unnecessary re-initialization of the LaymanAPI
in the layman sync plugin module.
argsparser.py: Adds --storage cli option
layman_.py: Adds subprocess use of --storage flag

In order to respect the location of the repository defined by repos.conf
the subprocess now specifies the storage location.
layman_.py: Overrides default storage value for API

The default storage value will be overwritten upon initialization
of the LaymanAPI in order to respect the location reported by
repos.conf.
reposconf.py: Adds sync-type checking
layman_.py: Adds checks to ensure repo location

Previously it was made so that the initialization of the LaymanAPI
wouldn't occur if it already existed. This could however lead to
reporting the incorrect storage location for repositories reported
by repos.conf. Checks have been added to ensure that the LaymanAPI
will be re-initialized if the storage location reported by repos.conf
differs from what was already previously reported.