Set current working directory for VOLTTRON and VOLTTRON agents

[schandrika]

Is your feature request related to a problem? Please describe.
When VOLTTRON or agents create files in a configured path, then relative paths are relative to where volttron was started from.
This could cause the issue in several situations for example

1. When more than one instance of volttron is started from the same location with the command VOLTTRON_HOME=<some path> volttron -vv -l volttron.log & at say different point in time
2. When sqlitehistorian is run with a configuration pointing to a relative path or with just a file name without path

Describe the solution you'd like
Set current working directory of VOLTTRON process to $VOLTTRON_HOME. Set current working directory of agents $VOLTTRON_HOME/agents/

Describe alternatives you've considered
We could always use absolute path to avoid this issue but that is not very convenient
Additional context
When porting agents from monolithic. Care should be taken to verify relative paths in configuration. But modular version of VOLTTRON is not backward compatible with monolithic code, we can simply add it to porting instruction