hvplot ignores default legend_position

#### ALL software version info
bokeh 2.3.0
holoviews 1.14.2
hvplot 0.7.1

#### Description of expected behavior and the observed behavior
Hello,
when setting default values for the legend position in holoviews, I expect it to be also respected by hvplot, if not specified otherwise in the hvplot call
```opts.defaults(opts.NdOverlay(show_legend=True, legend_position='top_left', legend_cols=False))
df.hvplot()
```
but the code above plots the legend at the standard hvplot position on the right and not on the specified default position.

#### Complete, minimal, self-contained example code that reproduces the issue

```
import holoviews as hv
import hvplot.pandas
import pandas as pd
from holoviews import opts
import pandas.util.testing as tm

opts.defaults(opts.NdOverlay(show_legend=True, legend_position='top_left', legend_cols=False))

df = tm.makeDataFrame()
plt = df.hvplot()
hv.save(plt,"test.html")
```