Docs for TOOLS-1969: Set mongoreplay play host via environment variable

Description

Description:

If MONGOREPLAY_HOST is set in the environment, it replaces the default value for the --host argument.

Engineering Ticket Description:

For security reasons, customers would like to be able to provide a URI with user/password credentials without putting them on the command line. A minor change to the flags options for "--host" will allow setting the value via the MONGOREPLAY_HOST environment variable.

Scope of changes

Impact to Other Docs

MVP (Work and Date)

Resources (Scope or Design Docs, Invision, etc.)