Call mod_init when invoking state.single

This ensures that mod_init is run when state.single is invoked.
salt.state.State.call_chunk() is what runs mod_init, but call_chunk() is
not invoked by state.single since requisites do not need to be resolved.

This commit adds a call to salt.state.State._mod_init() just before
salt.state.State.call() is invoked, making sure that the state's
mod_init() function is called like it is in state.sls and
state.highstate.
Merge pull request #11182 from terminalmage/state-single-mod-init

Call mod_init when invoking state.single